A really simple fix for Copy would be to limit it to <=2 CMC creatures.
Truth I think is fine. People aren't going to forget because when you play it they are going to put their hand on the table and then they won't pick it up.
I haven't seen much of the rest of your set, but just judging these cards in a vacuum:
A lot of these cards are really cool. Nature/Nurture is a really nice card that can help in a lot of bad situations. Truth/Dare and Finders/Keepers are really interesting.
A couple of these are really OP:
-Live Long/Prosper is insane. Zombify and Explosive Vegetation are fine cards on their own, and Prosper just searches for any land card, so you can get 2 cloudposts, or wolf run/inkmoth or something.
-Copy/Paste - I think even if Copy was a card without the other side, it would probably be banned in a lot of standard formats. Gearhulks, Sundering Titan, Reveillark etc. really break it.
-Fame/Fortune - this might be OK, but since you will only ever use Fame as a hard removal card and otherwise you still have Fortune which is actually pretty disruptive and can easily help with graveyard synergies, one or both sides are undercosted. One on it's own is totally fine, but that's mainly because Fame is too niche to play without another option. I think Fortune is likely enough to be good in the right deck that the opportunism of just hosing a duplicate permanent is insane (esp. in a color that normally can't do that) will be really strong when it comes up.
-I'm not a huge fan of Cruel/Unusual because I think Infest is good enough on it's own. You made the red side of it really narrow (and usually bad) to compensate, which I don't think is good practice.
I actually think Basket of Light is pretty damn strong, probably gains you at least 5 life a turn and can be played outside traditional life gain colors.
Basket of Darkness originally only exiled 1 card per turn and cost 1 - basically it was supposed to be a balanced Aether Vial. I decided that you would probably never cheat out anything that cost more than 2, so I decided to make it 2 cards and cost 3 mana. The card loss might be too much of a drawback though - maybe it should cost 2 mana? Or a somewhat janky solution is to give the player the option of 1 or 2 cards (or more?). Either way this card sort of messes with some fundamental relationships between mana and cards that I think you would have to playtest to get right.
Basket of Light 4
Legenary Artifact
At the beginning of your upkeep, tap another permanent you control. T: Gain life equal to the number of tapped permanents you control.
Basket of Darkness 3
Legendary Artifact
At the beginning of your upkeep, exile two cards from your hand. T: You may cast a spell exiled with ~ without paying its mana cost if X or more cards are exiled with ~, where X is the spell's converted mana cost.
Basket of Pursuit 6
Legendary Artifact
At the beginning of your upkeep, return a creature you control to its owner's hand. T, reveal a creature card from your hand: Target creature gains all abilities of the revealed card.
You would probably want to add a mana cost to the activated ability to avoid infinite combos. Another kind of cool option would to have the activated ability be "Tap 2 untapped wizards" so it takes some work to get the inspired effect.
It's essentially taking Broodmate Dragon and adding flexibility (in the form of easier color combos and many options based on matchup) and power in exchange for 1 more CMC.
Stitcher's Reproduction copies creatures in play FYI. I think the cost is fine. It's tough to find a way to specifically break the transform effect but there are definitely a few things you can do with the transform effect. Obviously you're assuming it would be played in a transform set so there would be something to transform.
Incineration Giant is pretty simple but would create a lot of interesting decisions and it has a pretty good combo of versatility and power without being OP. It's also pretty interesting because it gives red an expensive card with some late game punch but specifically is bad against big creatures.
Argent Lance 2
Artifact - Equipment (U)
Exlated
Equip 3
Equipped creature getes +1/+2 and has vigilance.
The main thing I'm worried about with making this a common is that if you mill yourself it might be a bit counterintuitive. Although it seems like all the other kindle effects go at common, probably for the reason you guys discussed.
As far as limited goes it obviously depends on the rest of the format. I think even if you draft 4 of these you're probably not going to have a very cohesive strategy without other cards to support it - you're probably going to draw 2 or less each game so you just mill your opponent for 12.
Brain Drain 1U
Sorcery (U)
Target player puts the top 4 cards of his or her library into his or her graveyard, then puts the top 4 cards of his or her library into his or her graveyard for each card named Brain Drain in your graveyard.
If people are seriously trying to break this format you would have to aggressively ban. As it is there are a lot of potential OTK strategies - it's probably even as simple as draw your entire deck with blue draw spells, turnabout to untap and banefire to kill your opponent.
"Recover" exists. Controvert; I think you mean from the grave but I don't see how this interacts well with much of anything in AMmnkhet.
Haha that's the one I actually spent time thinking about the name, so of course I found one that's been used. I was going to call it "Excavate" but I thought that would make it sound like you were getting from the graveyard (an effect I apparently achieved anyway).
I meant it would return something from the battlefield. This one is pretty narrow but I thought it work with Trials and Cartouches. I don't think it's really a great mechanic, except maybe as a single cycle or something. The Indiana Jones flavor makes more sense if you're digging some ancient artifact out of the ground anyway though.
Does this set have a lot of potential "B"s? Is there something in particular that you either want to encourage or happens to occur in a lot of cards in your set?
I'm not sure what you're trying to argue exactly, but I was just saying that the thing that got Seething Song (which I don't think is OP) banned in modern would not apply to this card.
It is also worth mentioning that this card is a lot easier to play in a deck like storm where you have a lot of blue deck manipulators - seething song requires you to convert the mana via manamorphose or have blue left over which is somewhat significant and I think offsets the time drawaback. It's also great against Remand.
Truth I think is fine. People aren't going to forget because when you play it they are going to put their hand on the table and then they won't pick it up.
A lot of these cards are really cool. Nature/Nurture is a really nice card that can help in a lot of bad situations. Truth/Dare and Finders/Keepers are really interesting.
A couple of these are really OP:
-Live Long/Prosper is insane. Zombify and Explosive Vegetation are fine cards on their own, and Prosper just searches for any land card, so you can get 2 cloudposts, or wolf run/inkmoth or something.
-Copy/Paste - I think even if Copy was a card without the other side, it would probably be banned in a lot of standard formats. Gearhulks, Sundering Titan, Reveillark etc. really break it.
-Fame/Fortune - this might be OK, but since you will only ever use Fame as a hard removal card and otherwise you still have Fortune which is actually pretty disruptive and can easily help with graveyard synergies, one or both sides are undercosted. One on it's own is totally fine, but that's mainly because Fame is too niche to play without another option. I think Fortune is likely enough to be good in the right deck that the opportunism of just hosing a duplicate permanent is insane (esp. in a color that normally can't do that) will be really strong when it comes up.
-I'm not a huge fan of Cruel/Unusual because I think Infest is good enough on it's own. You made the red side of it really narrow (and usually bad) to compensate, which I don't think is good practice.
I actually think Basket of Light is pretty damn strong, probably gains you at least 5 life a turn and can be played outside traditional life gain colors.
Basket of Darkness originally only exiled 1 card per turn and cost 1 - basically it was supposed to be a balanced Aether Vial. I decided that you would probably never cheat out anything that cost more than 2, so I decided to make it 2 cards and cost 3 mana. The card loss might be too much of a drawback though - maybe it should cost 2 mana? Or a somewhat janky solution is to give the player the option of 1 or 2 cards (or more?). Either way this card sort of messes with some fundamental relationships between mana and cards that I think you would have to playtest to get right.
Legenary Artifact
At the beginning of your upkeep, tap another permanent you control.
T: Gain life equal to the number of tapped permanents you control.
Basket of Darkness 3
Legendary Artifact
At the beginning of your upkeep, exile two cards from your hand.
T: You may cast a spell exiled with ~ without paying its mana cost if X or more cards are exiled with ~, where X is the spell's converted mana cost.
Basket of Pursuit 6
Legendary Artifact
At the beginning of your upkeep, return a creature you control to its owner's hand.
T, reveal a creature card from your hand: Target creature gains all abilities of the revealed card.
It's essentially taking Broodmate Dragon and adding flexibility (in the form of easier color combos and many options based on matchup) and power in exchange for 1 more CMC.
Incineration Giant is pretty simple but would create a lot of interesting decisions and it has a pretty good combo of versatility and power without being OP. It's also pretty interesting because it gives red an expensive card with some late game punch but specifically is bad against big creatures.
Argent Lance 2
Artifact - Equipment (U)
Exlated
Equip 3
Equipped creature getes +1/+2 and has vigilance.
One kind of cool scenario for Growing Brilliance is you can put it on a genju and just get the effect whenever you activate the land.
As far as limited goes it obviously depends on the rest of the format. I think even if you draft 4 of these you're probably not going to have a very cohesive strategy without other cards to support it - you're probably going to draw 2 or less each game so you just mill your opponent for 12.
Sorcery (U)
Target player puts the top 4 cards of his or her library into his or her graveyard, then puts the top 4 cards of his or her library into his or her graveyard for each card named Brain Drain in your graveyard.
I meant it would return something from the battlefield. This one is pretty narrow but I thought it work with Trials and Cartouches. I don't think it's really a great mechanic, except maybe as a single cycle or something. The Indiana Jones flavor makes more sense if you're digging some ancient artifact out of the ground anyway though.
Foretold <cost> (You may cast this card for it's Foretold cost if a card you own was exiled from anywhere this turn.)
Wounded - As long as ~ has a -1/-1 counter on it, <effect>.
Panic (Whenever you discard a card ~ gets +1/+1 until end of turn.)
Recover (Whenever ~ enters the battlefield, you may return target artifact or enchantment you control to it's owner's hand)
It is also worth mentioning that this card is a lot easier to play in a deck like storm where you have a lot of blue deck manipulators - seething song requires you to convert the mana via manamorphose or have blue left over which is somewhat significant and I think offsets the time drawaback. It's also great against Remand.