Happy Holidays everyone! I'm new here, but I enjoy making custom MtG cards so I figure my first post should be some card designs.
Creature - Elf Merfolk Mutant
Adapt 1 UG
Remove a +1/+1 counter from a creature you control: Target creature gains your choice of Flying, Trample, or Hexproof until end of turn.
Creature - Demon
If you cast ~ for its Spectacle cost, ~ enters the battlefield with X +1/+1 counters on it, where X is equal to the amount of life lost by players this turn.
Exotic Experimentalist - This does quite a lot; there are a bunch of decisions to make here and that can be tough to keep track of. The design's not THAT overcomplicated but, simplify where you can. Maybe "Remove a +1/+1 counter from a creature you control: THAT CREATURE gains..."
Rabid Ringleader - Super reasonable, but small for a Demon. Make it a Devil.
It may seem pedantic, but that's the way they formatted it on the cards. I doubt they're going to take advantage of it, but structuring it this way does allow for some broader exploration of the keyword than just a boring Monstrosity variant.
Exotic Experimentalist is exactly the kind of thing Adapt is going to need to make any inroads in constructed. It's something Simic has done before in the Graft days, so I have high hopes for it, but we'll need to wait and see. I think the issue is really that Trample, Flying, and Hexproof are not of equal value. Plaxcaster Frogling and Helium Squirter have different activation costs. I would follow their lead and put a mana component to the second ability as well. I'd place this at uncommon.
I agree with Void_Nothing, Rabid Ringleader feels more Devilish than Demonic. You've got a pretty threatening draft rare here.