I do like the idea of Acid Rain being a move but I think that doing double damage on Grass-types considering Grass is weak to Poison and on a Ludicolo for example, it will do 8x damage due to the weakness of Grass to Poison and the 2x damage bonuses for Ludicolo being both a Grass and Water type, not saying that Whimsicott counts because it is naturally double weak to Poison because of its Grass/Fairy typing. Perhaps Game Freak can mitigate this effect by making Acid Rain have the additional effects of being super effective on Water and Steel.I'm still waiting for Acid Rain to be come a real move. I'm honestly surprised it hasn't happened yet.
It would just be a Poison-type weather effect that damages pokemon each turn, and deals double damage to Water, Grass, and Steel-types. It would be a real game changer, for sure.