Got one for Heliolisk I've been thinking about for a bit. Heliolisk Crest: Summons weather based on its ability.
For those not aware, all of Heliolisk's abilities are based on weathers (in a way), Dry skin (Rain), Solar Power (Sun), Sand Veil (Sandstorm). Could also give a small stat increase if you'd like.
OR
Instead of summoning a weather effect it could double down on needing a weather effect. Heliolisk Crest: Adds an ability based on its current ability. Dry Skin (Swift Swim or Rain Dish), Solar Power (Chlorophyll or Leaf Guard), Sand Veil (Sand Rush or Sand Force? (Sand force doesn't really work because Heliolisk doesn't get any steel/ground moves but I needed SOMETHING to put as an alternative otherwise it'd be weird.)
OR
Instead of those we give it the last weather effect it was missing Heliolisk Crest: Changes Heliolisk's ability to Ice Body/Snow Cloack/Slush Rush during Hail.
If it wasn't obvious Heliolisk is one of my fav pokemon and I love the idea of adding more weather effects to it.