Here are my probably horrible ideas for some crests: Flareon crest: immediately burns the holder upon entry and gives it a ground resistance
Runegrigus crest: Cofagrigus but physical Decidueye crest : contact moves now disable the last move used by the opponent Purrseker crest : "claw" moves have +1 priority Flapple crest: ignores hustle accuracy drop, but lowers attack by 10 Appletun crest: becomes neutral to ice moves, but gives it another weakness to poison moves Samurott crest: slashing moves (razor shell included) lower the opponent defense by one stage Zebstrika crest: adds fire type to its original typing and normal-type moves become fire-type Mr Mime crest: upon switching in, mimic automatically becomes the last move used by the opponent Quagsire crest: makes it also unaware of the field changes Sandslash/Alolan Sandslash crest: activates sand rush/slush rush upon entry