Had some silly ideas during work, so I decided to drop'em here:
Mimikyu Crest: Disguise never breaks, but still takes 1/8 damage from getting hit and heals 1/8 of Max HP if the holder is not hit this turn, but the holder can only use status moves.
Wobbuffet Crest: Decreases the opponent's highest attacking stat by one stage and everytime the holder is not hit by a move that does damage, activates all entry hazards at the end of the turn;
Slaking Crest: If the holder uses Slack Off, Truant gets supressed next turn;
Oranguru Crest: Upon using Instruct, increases the target's highest stat by one stage and heals the holder by 1/8;
Passimian Crest: Ignores Follow Me and Spotlight's effects;