Fog Machine - Brings back the fog weather effect from Gen 4. Half the new Pokemon have it as their only ability. Players quit in droves.
Oh Arceus noooo!
But for serious, I wouldn't mind more Lightningrod-esque abilities for the otherwise weak new Pokemon. I mean, heck, Storm Drain made Gastrodon super useful in countering rain teams, why not an ability like that for more weaker Pokemon?
Perhaps an ability that increases accuracy to 100% for damaging moves but puts in a power penalty. Inaccurate moves like DynamicPunch could see limited use outside of No Guard.
Perhaps a randomized and limited Wonder Guard? Perhaps every turn the ability activates and renders the Pokemon immune to blank-type moves for that turn. Maybe a bit subject to RNG, but so is Moody, and it could be interesting.
A reverse Multi-scale, making Pokemon below say... 40% health take less damage. Do you go for an HP balancing act and go for a set with perhaps Reversal or do you play it safe and try to stay healthy?
I think those would be pretty interesting to see.