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 Ice-type, cannot miss, and effects of Present are now 1.5x (damage and healing)
Slaking Crest - All stats are reduced by 20%, replaces Truant with Gluttony
Flygon Crest - Replaces Levitate with Punk Rock, changes Normal-type moves to Ground-type
Luxray Crest - Biting moves increase the user's speed one stage
Vanilluxe Crest - In hail, loses 1/8 HP per turn but boosts Sp. Atk by 1.25x (hail version of Solar Power but weaker)
Komala Crest - Contact moves have a 33% chance of inflicting Drowsy on the target
Druddigon Crest - Reduces physical damage taken by 25%, triggering Rough Skin also reduces the opponent's defense by one stage