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 first move
noivern crest: normal-type moves become dragon-type and/or all physical flying-type moves become special.
[EDIT] thought of an additional crest idea, so sorry if these have been thought of already!
lilligant crest: adds dancer ability to the user. when user is sent out, quiver dance is activated regardless of whether or not the move is in lilligant's moveset.
[EDIT2] oops i did it again
staraptor crest: gives fighting type moves STAB / gives the user it's alternative ability in addition to the one it already has (adds intimidate onto a reckless staraptor or adds reckless onto an intimidate staraptor)