I think Unearth is coming back in Eldritch Moon. Why? Because it's like Flashback but different.
Let me explain. First, Flashback is the perfect fit for this block because it makes discard outlets work better without Madness cards in hand. It also goes with the incidental mill found in Shadows Over Innistrad. Flashback and Madness were both originally designed as part of the same block and were meant to play well together.
Next, why not Flashback? Flashback was in the original Innistrad, and wanting Shadows Over Innistrad to not be too similar has been cited as a reason to not bring it back again.
Finally, why Unearth? Because Unearth is just Flashback on creatures. It makes sense here for similar reasons but is also semantically different enough that Wizards might feel comfortable bringing it back. For what it's worth, Unearth is a good mechanic that was probably underutilized the first time around because it was designed as a faction mechanic rather than a set mechanic, and it's been almost a decade since then anyway so it's due for a revisit in the near future regardless.
Starting in Theros, all monocolor sets (except Shadows Over Innistrad) have all had a single uncommon for each 2-color combination that pretty explicitly states what a deck of those 2 colors is supposed to be doing in limited. A quick search for these cards should tell you all you want to know.
A better question would be "which sets are well-designed?". As a rule of thumb, the well-designed sets are the ones where a good deck's direction is not decided by its colors.
There's a lot I like about Dominion (even though it can be hard to come back from being behind on lands), but it has a fundamental flaw that makes it unusable: whoever goes first gets a huge and unfair advantage. In normal games where both players play a land every turn and don't play into any land/ramp themes, whoever goes first would have Dominion active on their turn and the second would never have it.
Pacifism and Reduce in Stature are actually not good when played against decks with exploit; they're still good cards but I'd rather pick a good creature over either.
Dromoka Dunecaller is much worse than it looks. It doesn't actually block anything relevant, 2 mana to tap things is not great (especially in what appears to be a mana-intensive format), and it can't tap fliers, which are often what you'll want to tap the most. It also does not play well with bolster because every time you are forced to bolster onto it you have effectively lost a bolster trigger. I would actually rather have a Dragon's Eye Servant than this if I'm in the market for a defensive white creature because it blocks better for less mana and can make some use out of a +1/+1 counter if it gets bolstered. (Neither card actually looks good)
Elusive Spellfist has to be one of the top blue commons. It's cheap, it blocks (can block a morph), and does 2 damage whenever you play a noncreature -- 4 if that spell rebounds. Kiln Fiend + Distortion Strike was incredibly powerful in RoE drafts; this is a Kiln Fiend with less power but turns all of your spells into Distortion Strikes, and it blocks small things in the meanwhile.
Artful Maneuver is also good card; it saves your creature one turn, then delivers a whooping the next. And it goes great with Elusive Spellfist. Drafting as many of these 2 cards as you can get your hands on and taking the best substitutes available when you can't sounds like a great way to draft UW.
Taigam's Strike is just a bad Distortion Strike but it's still a good card for aggressive blue decks and will end games early. Elusive Spellfist is very powerful for similar reasons; it can do serious damage in any spell-oriented blue deck while also blocking early.
Dutiful Attendant is a trap; it enables exploit somewhat but you won't always get value out of the effect, it is otherwise useless outside of chump blocking (0/2 is not a useful blocker), and there's no need for 3 drops in a morph format. Palace Familiar, on the other hand, is the real deal, as are Jeskai Sage and the obviously strong Youthful Scholar.
Remember Aven Skirmisher? It can be quite good now in a very bolster-heavy deck.
The best way to draft Zendikar block is to open all your Zendikar/Worldwake packs without drafting them, then draft triple RoE. Zen/Zen/Worldwake is one of the most hated draft formats of all time because the optimal strategy is to pick aggressive 2 drops over anything else. Triple RoE, on the other hand, is among the best draft formats of all time.
The best thing you could do with your packs triple Rise of the Eldrazi drafts. The worst thing you could do with your packs is squander your packs on some unholy mix of RoE and Zendikar in the same draft.
Similarly, Shadowmoor block is best enjoyed without the Eventide packs -- even the lead designer of Eventide has admitted that Eventide messed with Shadowmoor draft too much and hurt the draft format. Triple Eventide is also notoriously bad, mostly due to the mimics but also because there's nothing good about it. Go triple Shadowmoor.
For Lorwyn, conventional wisdom says go Lorwyn/Lorwyn/Morningtide because that's the official format and it's a good format and you have the packs. That said, triple Lorwyn is more popular by a small margin (you can make Elvish Handservant decks), so you might want to go triple Lorwyn. I've played Morningtide sealed before and found it to be fun (I had 2 Bitterblossoms, of course I found it fun) but prowl decks are probably overpowered in triple Morningtide. I'd say you should go Lorwyn/Lorwyn/Morningtide to get the most out of your packs, but also consider 3xLorwyn.
These look really good, but I have a few issues with them:
I don't like how advise is just a keyword for looting-- and if you do keyword the loot effect, it feels weird to have it called something other than "loot." Whipping has too much rules text.
A keyworded ability is generally something you would want to try to build a deck around. A lot of these mechanics are not.
Sit-in is going to feel really weak because it stops working after the first few turns. How about if you change it to "3 or fewer untapped lands"?
There probably isn't any room for it, but if you do another set I think it would be interesting to have a theme of cards that get better when you have over X lands-- it would make the decision to throw extra lands away to the various effects of the set more interesting. Or it might clash too much with them and suck. I don't know which.
Another way to make the decision to throw extra lands away more involved is to make really cool lands!
Pearls have been associated with white before, so I can see how Pearl Diver cares about white cards. But the flavor doesn't justify it caring about black cards.
The set description discusses treaties being constantly made and broken, but the alliances seem pretty clear to me--each color is allied with its allied colors and not with its enemy colors. The theme of shifting alliances should be clear throughout the set if it is indeed one of your themes. (Splash is not enough)
The list of purple abilities here is just a combination of blue and black things. It doesn't justify why purple deserves its own color.
"Strange bedfellows" is a good point, and I think that purple should uphold it. The "strange" part of the relationship can work out nicely: green hates fliers, artifacts, and enchantments, and purple can like all of these things. Artifacts (and, to a lesser extent, enchantments) can represent artificial change. Levitation would be a nice card to shift into purple.
I'd make purple the main flying color because it contrasts nicely with green, I can't think of a reason why blue should be the main flying color rather than purple, and blue already has some evasion in the form of unblockability and Sleep-type effects that are increasingly common these days. (note that I wouldn't take all the fliers away from blue) Temporary flying ("P: ~ gains flying until end of turn") is also a nice way of showing progress, and I like the counter-accumulating Chronomaton idea. also Time Warp.
Counterspells and discard do not make sense in purple. They hinder the opponent's progress but do nothing for your own. That's why the "bedfellows" part of the green-purple relationship is important-- purple should have mana acceleration to represent progress. Deranged Assistantwithout the self-mill clause and a nonbasic land ramp spell would give purple its own way of ramping distinct from green's.
The "purple needs counters and discard" idea is probably a result of those effects being restricted so much to specific colors, and people want them in more colors.
The "strange bedfellows" solutions: give red a counter, 1R, counter spell unless its controller lets ~ deal 4 damage to him/her. give white life payment effects that benefit your creatures. give white hand disruption, Tidehollow Sculler-style.
So I think purple deserves ramp that feels different from green's, and blue's fliers and artifact-loving creatures, and possibly enchantment support / anything else that represents progress/change/development but should not get counters/discard. Blue loses things but still has evasion, counters, bounce, mind control, and probably more tempo cards than it used to.
Predate is all downside, that's no fun. Why would I want to have a deck full of predate cards?
Hunting Fodder (which is also all downside, even with predate) doesn't fix the problem because why should I have to play that to make my other cards playable? And if I attack and they block with a goat (and it's their last creature) and it dies, then so do all my creatures. Not fun.
Abilities should be all upside because that's more fun. And "morbid" seems to fit your theme pretty well-- it rewards you for killing their things, but it triggers when your creatures die, so you can sacrifice your own things to ensure that it triggers if necessary.
And whether you take my advice or not, check out the "hunted" cycle from Ravnica.
Kuruk Shrine looks like it should be white. Blue tap effects are cheaper and not repeatable, whereas white ones do it every turn. Also, shrines seem like more of a white thing unless you have cool art / flavor text to go with it that justify the blue.
Smokescreen's name sounds either red or black, while its effect is definitely green (Ranger's Guile).
Let me explain. First, Flashback is the perfect fit for this block because it makes discard outlets work better without Madness cards in hand. It also goes with the incidental mill found in Shadows Over Innistrad. Flashback and Madness were both originally designed as part of the same block and were meant to play well together.
Next, why not Flashback? Flashback was in the original Innistrad, and wanting Shadows Over Innistrad to not be too similar has been cited as a reason to not bring it back again.
Finally, why Unearth? Because Unearth is just Flashback on creatures. It makes sense here for similar reasons but is also semantically different enough that Wizards might feel comfortable bringing it back. For what it's worth, Unearth is a good mechanic that was probably underutilized the first time around because it was designed as a faction mechanic rather than a set mechanic, and it's been almost a decade since then anyway so it's due for a revisit in the near future regardless.
A better question would be "which sets are well-designed?". As a rule of thumb, the well-designed sets are the ones where a good deck's direction is not decided by its colors.
Dromoka Dunecaller is much worse than it looks. It doesn't actually block anything relevant, 2 mana to tap things is not great (especially in what appears to be a mana-intensive format), and it can't tap fliers, which are often what you'll want to tap the most. It also does not play well with bolster because every time you are forced to bolster onto it you have effectively lost a bolster trigger. I would actually rather have a Dragon's Eye Servant than this if I'm in the market for a defensive white creature because it blocks better for less mana and can make some use out of a +1/+1 counter if it gets bolstered. (Neither card actually looks good)
Elusive Spellfist has to be one of the top blue commons. It's cheap, it blocks (can block a morph), and does 2 damage whenever you play a noncreature -- 4 if that spell rebounds. Kiln Fiend + Distortion Strike was incredibly powerful in RoE drafts; this is a Kiln Fiend with less power but turns all of your spells into Distortion Strikes, and it blocks small things in the meanwhile.
Artful Maneuver is also good card; it saves your creature one turn, then delivers a whooping the next. And it goes great with Elusive Spellfist. Drafting as many of these 2 cards as you can get your hands on and taking the best substitutes available when you can't sounds like a great way to draft UW.
Dutiful Attendant is a trap; it enables exploit somewhat but you won't always get value out of the effect, it is otherwise useless outside of chump blocking (0/2 is not a useful blocker), and there's no need for 3 drops in a morph format. Palace Familiar, on the other hand, is the real deal, as are Jeskai Sage and the obviously strong Youthful Scholar.
Remember Aven Skirmisher? It can be quite good now in a very bolster-heavy deck.
I'm glad that Seismic Rupture exists. I miss Pyroclasm; this is close enough.
There are enough relevant enchantments here and in FRF to make Ainok Survivalist good.
The best thing you could do with your packs triple Rise of the Eldrazi drafts. The worst thing you could do with your packs is squander your packs on some unholy mix of RoE and Zendikar in the same draft.
Similarly, Shadowmoor block is best enjoyed without the Eventide packs -- even the lead designer of Eventide has admitted that Eventide messed with Shadowmoor draft too much and hurt the draft format. Triple Eventide is also notoriously bad, mostly due to the mimics but also because there's nothing good about it. Go triple Shadowmoor.
For Lorwyn, conventional wisdom says go Lorwyn/Lorwyn/Morningtide because that's the official format and it's a good format and you have the packs. That said, triple Lorwyn is more popular by a small margin (you can make Elvish Handservant decks), so you might want to go triple Lorwyn. I've played Morningtide sealed before and found it to be fun (I had 2 Bitterblossoms, of course I found it fun) but prowl decks are probably overpowered in triple Morningtide. I'd say you should go Lorwyn/Lorwyn/Morningtide to get the most out of your packs, but also consider 3xLorwyn.
I don't like how advise is just a keyword for looting-- and if you do keyword the loot effect, it feels weird to have it called something other than "loot." Whipping has too much rules text.
A keyworded ability is generally something you would want to try to build a deck around. A lot of these mechanics are not.
Sit-in is going to feel really weak because it stops working after the first few turns. How about if you change it to "3 or fewer untapped lands"?
There probably isn't any room for it, but if you do another set I think it would be interesting to have a theme of cards that get better when you have over X lands-- it would make the decision to throw extra lands away to the various effects of the set more interesting. Or it might clash too much with them and suck. I don't know which.
Another way to make the decision to throw extra lands away more involved is to make really cool lands!
The original twobrid cycle, however, is very specific to Archester and features a variety of objects and effects relevant to this set's themes.
The set description discusses treaties being constantly made and broken, but the alliances seem pretty clear to me--each color is allied with its allied colors and not with its enemy colors. The theme of shifting alliances should be clear throughout the set if it is indeed one of your themes. (Splash is not enough)
Headache R
Instant
Any player may have ~ deal 3 damage to him or her. If no one does, counter target spell.
"Strange bedfellows" is a good point, and I think that purple should uphold it. The "strange" part of the relationship can work out nicely: green hates fliers, artifacts, and enchantments, and purple can like all of these things. Artifacts (and, to a lesser extent, enchantments) can represent artificial change. Levitation would be a nice card to shift into purple.
I'd make purple the main flying color because it contrasts nicely with green, I can't think of a reason why blue should be the main flying color rather than purple, and blue already has some evasion in the form of unblockability and Sleep-type effects that are increasingly common these days. (note that I wouldn't take all the fliers away from blue) Temporary flying ("P: ~ gains flying until end of turn") is also a nice way of showing progress, and I like the counter-accumulating Chronomaton idea. also Time Warp.
Counterspells and discard do not make sense in purple. They hinder the opponent's progress but do nothing for your own. That's why the "bedfellows" part of the green-purple relationship is important-- purple should have mana acceleration to represent progress. Deranged Assistant without the self-mill clause and a nonbasic land ramp spell would give purple its own way of ramping distinct from green's.
The "purple needs counters and discard" idea is probably a result of those effects being restricted so much to specific colors, and people want them in more colors.
The "strange bedfellows" solutions: give red a counter, 1R, counter spell unless its controller lets ~ deal 4 damage to him/her. give white life payment effects that benefit your creatures. give white hand disruption, Tidehollow Sculler-style.
So I think purple deserves ramp that feels different from green's, and blue's fliers and artifact-loving creatures, and possibly enchantment support / anything else that represents progress/change/development but should not get counters/discard. Blue loses things but still has evasion, counters, bounce, mind control, and probably more tempo cards than it used to.
Hunting Fodder (which is also all downside, even with predate) doesn't fix the problem because why should I have to play that to make my other cards playable? And if I attack and they block with a goat (and it's their last creature) and it dies, then so do all my creatures. Not fun.
Abilities should be all upside because that's more fun. And "morbid" seems to fit your theme pretty well-- it rewards you for killing their things, but it triggers when your creatures die, so you can sacrifice your own things to ensure that it triggers if necessary.
And whether you take my advice or not, check out the "hunted" cycle from Ravnica.
Smokescreen's name sounds either red or black, while its effect is definitely green (Ranger's Guile).