I read your series today and I really liked them. Your opinions are well supported, and your analysis covers each keyword from many different aspects in a broad view.
I have one correction to make though. Suspend wasn't the first ability to use the exile zone in a way that no other card could, imprint in Mirrodin block did it first (albeit in an unexciting and not very memorable way). This mechanic still has quite a lot of untapped design space.
I don't have an idea what the first keyword would be. Trample is one of the most beloved mechanics of all time, that plays well and has history. However, it's not very innovative - many attacking/blocking mechanic have been made alongside it from the very beginning, and it's not as resonant as flying.
OTOH I definitely know what will make the worst ten keywords of all time: Shroud!
Krasis >> Nissa
Scavenger > Carver > Engineer
Kiora Bests the Sea God > Frost Titan
Galvanic Blast > Mizzium Mortars
Blast > Staggershock, 3 mana burn just isn't too appealing.
Horizon > Pathways > bicycle lands
1. Flooded Strand
2. Tundra
3. Hallowed Fountain
4. Celestial Collonade
5. Adarkar Wastes
6. Hengegate Pathway
7. Seachrome Coast
8. Irrigated Farmland
9. Glacial Fortress
Dimir:
1. Polluted Delta
2. Underground Sea
3. Watery Grave
4. Creeping Tar Pit
5. Underground River
6. Clearwater Pathway
7. Darkslick Shores
8. Fetid Pools
9. Drowned Catacomb
Sovereign > Paladin > Spirit - Paladin trades with everything and does nothing if you don't attack with it. Sovereign has a strong static ability in your worst matchups.
Fireblast > Stoke > Char - Fireblast is narrow, but far stronger. The two damage of Char proved quite bad against aggro.
I'll probably test showdown at some point, but aggro is saturated with four drops in those colors. Two-drops have a much milder competition.
Forsake > Fragmentize - Forsake is maindeckable, Fragmentize is a narrow sideboard card in most cubes. It also sucks to draw it a turn after they Tinkered their Myr Battlesphere. If artifacts are so prevalent Fragmentize is maindeckable, go with that.
Rishadan Port also underperformed for us for some time.
We cut Armageddon and Ravages for being too win-more, I am happy about this decision. I remember the times where you could go one drop, two drop, three drop, and Armageddon to a win, but these days are long gone. It is much harder to have a significant advantage by turn four. If the game didn't go significantly in your favor, then almost any other card is preferable. Plus,
Reveillark we cut ages ago, it is narrow and slow. I don't think the power level is high enough to justify its shortcomings.
At 720, I play the cheat effects you have listed to good effect.
Balance is very powerful but has a few drawbacks. One, it is a build-around; it is very likely that no one will play it if it was opened pack three. Two, it is legitimately a hard card to play correctly even in the "right" deck. I can see playgroups where it would not be a good fit.
The card doesn't count creature tokens. Sorry to be a party pooper
How often will it nuke something? It needs to be a tapped creature with 2 or less toughness