i thought i had other crest ideas, but the only one i could remember has already been mentioned, while giving the intended pokemon an additional buff (flareon crest, which i would absolutely love). managed to think of at least a few additional ones though.
pikachu or kanto raichu crest: user's attacks will inflict either physical or special damage, whichever will be more effective. this could kind of be a throwback to the pre-physical/special split days.
decidueye crest: gives flying type moves STAB
zoroark crest: illusion stays intact after being hit by the