Oooh, would definitely love to see some less used Pokemon be made more viable! Some ideas I had:
Mightyena Crest - Using a sound-based move (i.e. Howl, Snarl, Roar) raises the user's Speed + Attack stat by one stage
Noivern Crest - Boosts Sp. Atk by 20%, on entry gives the Strong Winds weather effect
Manectric Crest - Swaps Atk and Sp. Atk, contact moves have an additional 15% chance for paralysis
Lanturn Crest - Boosts Def by 10%, stockpile grants effects of Aqua Ring and vice versa
Delibird Crest - Boosts Atk by 20%, Present is now