Gen 1:
Kabutops Crest: speed increase and sturdy ability
Hypno Crest: sp atk and atk increase with bad dreams or dream eater like ability (maybe add hypnosis 100% accuracy?)
Seaking Crest: speed increase and sap sipper ability
Snorlax Crest: can hit ghost types with normal moves and have leftover like healing
Tauros Crest: When using must recharge move like Giga impact or hyper beam, if opponent mon is knocked out, no need to recharge and tough claw (horn) ability.
Flareon Crest burned and speed increase and/or make guts like ability but increases sp atk instead o