what, nothing for the hallow? alright, then how about this.I don't know why I haven't thought of it before but :
Ichor Rain in harmdode crimson : inflicts ichor when outside and maybe some ichor monsters spawn on the surface.
Cursed Fallout in hardmode corruption : Looks similar to the stardust event, but with cursed fire with occasional flames falling. Cursed flames monsters spawn on the surface(maybe a special elemental too).
Honey rain in the jungle : slows you but gives honeyed buff, hornets spawn on the surface.
Toxic Vapors (in the lava cavern layer) : Gives you the poisoned debuff. Again, maybe a special elemental can spawn.
I may be starting to repeat number of ideas, but I forgot if they were already suggested
dazzling duststorm: you thought getting glitter in your eye was bad? try pixie dust. lowers visibility, inflicts confusion while you are in the hallow and makes illuminant enemies spawn above ground.