Here are some ideas that i can think of
Shedinja crest: gives shedinja disguise allowing it to survive weather changes like hail sandstorm/switch in to stealth rock
drapion crest: increase critical hit chance sniper drapion
Cacturne crest: Double the base speed and contact moves drain enemy hp
Noivern : Sound based moves gain flying typing. something like pixilate
Weezing: double damage caused by explosion