Zombies would turn into Skeletons, Meanwhile Slimes would turn into "Acidic Slimes" that deal acidic damage just by getting close to them, and they would also divide into more smaller slimes than usual as you fight them, which would be quite beneficial.
Also, in regards to gear giving protection to the acid rain, I think Gold armor should give high protection and take no durability damage from the rain, from the vanilla armors, Leather would give the highest protection, but would have it's durability degrading rather quickly by the acid rain.
Zombies would turn into Skeletons, Meanwhile Slimes would turn into "Acidic Slimes" that deal acidic damage just by getting close to them, and they would also divide into more smaller slimes than usual as you fight them, which would be quite beneficial. Also, in regards to gear giving protection to the acid rain, I think Gold armor should give high protection and take no durability damage from the rain, from the vanilla armors, Leather would give the highest protection, but would have it's durability degrading rather quickly by the acid rain.