Flareon Crest: Any damage dealing fire type move it uses raises its speed by 1 stage. If said move is Flame charge, then it gets +1 atk +1 speed
Espeon Crest & Umbreon Crest: Kinda like in Reborn Altforms; swap stats with each other, getting a defensive Espeon and an offensive Umbreon! Maybe they also become resistant to bug moves?
Flygon Crest: Someone already suggested this one, but STAB for bug moves.. pls... and also special moves deal damage according to its attack stat?
Aromatisse Crest: Restores HP when hit by a poison type move (I just want an excuse to use my plague doctor sprite edit on this thing jaja)
OR: Applies Poison Heal
Slurpuff Crest: Applies Cotton Guard upon entering the field
Noivern Crest: I think it's been suggested, but sound based moves deal x1.5 damage, and the holder becomes soundproof
Sylveon Crest: When the holder is hit by a contact move, the attacker becomes (or has a 50% chance of becoming?) infatuated. Could be with or without considering gender, depending on how broken it is idk
That's all I got fo now oof