Ok that's a stupid idea that I like to use in my Draft League match.
But make a Chandelure crest that gave him the Iron Ball effect (so divide the speed by 2) but setup Trick Room as it enter the field (this could be use by Nim/Lorna)
Other random idea from my past Draft League experience:
-Drapion crest: give him a focus energy on all his move (making a Sniper set really good). And maybe give some kind of boost in atk
-Hydreigon crest: Gave it parental bond for each head on special move but the 2 head only does 20%/30% of the first head damage (we still need to fight the dragon gym)
-Shaymin crest: give 100% accurate Seed Flare with the Serene Grace ability (even on the normal one while still having Natural cure) + a 1/16 heal recovery each turn
-Garbodor crest : activate a Perish Song but only for the opponent (because of the awful smell) also does 12% damage each turn to every non poison type mon (but this effect can still be pair with status or weather)
-Lanturn crest: gave it the eviolite effect or give it a 50% boost on every move that do light (so Dazzling Gleam, every electric move, signal beam, psybeam, etc)
-Bronzong crest: swap both atk/sp.atk into def/sp.def making it a slow offensif mon (also good on Nim trick room team)
I might have other idea but latter but that's it for now
