acid rain gradually applies poison up to 20 stacks (this is just base acid rain and JUST for acid rain you can stack poison from other sources).
P (posion) = T (time in acid rain affect (seconds))^A (Applied poison (+1)). Toxic Totality lets acid rain actually do damage according to the tier of TT.
Bubble shield protects against acid rain but will bubble shield up-time. Healing in acid rain cures the poison before healing the unit.
EX: a unit has 20 stacks of poison, you use healing ward, before healing the unit it will work on the poison.
Merics will insta heal poison form acid rain and healing, poison form other sources will have to suffer from the current ass meric heal system of curing then healing
I was thinking of adding "acid rain burns away enemy projectiles" but then i realized: i hate hard counters as much as i hate losing elo.
edit: probably over the top, makes acid rain op and could probbaly hard counter a low HP deck
edit 2: yeah its op with the ability to 20 stack poison but without that i think its fine