Hydreigon crest: when using a move like flamethrower the other two heads will do flamethrower as well but not the maximum power like 25% of it. (its a three attack middle head flamethrower 90 power other two heads 22,5 power separated)
Lycanrock crest: boost the speed and evasion when accelrock is used
Mimikyu Crest : When disguise fall his defense, attack and speed will be increased by 30%
Charizard crest: boost all stats when hit by a super effective move or critical hit
Togekiss crest: When switched in, it will boost one of its stat by 2 stages, if there is an ally, it will boost one of their stat by 1 stage aswell
Lucario crest: boost fighting type moves and his accuracy
Krookodile crest: doubles his attacks when there is a weather effect and when its over it turns back to normal
Kommo-o crest: when he gets a status effect, it will be removed instantly and his move will be boosted by 20%
Staraptor crest: when your opponent has placed an entry hazard staraptor nulifies it and boost its attack and speed
Ferrotorn crest: turns all the attack that are not the same as his type into grass or steel type and boost them for 30%
Avalugg crest: when summoned in a waterfield it will turn instantly in an icefield and it will turn back into an icefield even it they turn it back to a waterfield
Garchomp crest: his stats can’t be lowered by his opponent or by himself but when his opponent try to so all his stats will be boosted
Maybe put 3 type based crest at the beginning of the game like:
Flame crest: boost fire type pokemon there fire moves by 10%
Aqua crest: boost water type pokemon there water moves by 10%
Nature crest: boost grass type pokemon there grass moves by 10%
It can only be held by pokemon that have that type it is based on