Rain in the nether.
So this is the idea. Lava rain is an uncommon occurrence in the nether. It drips from the tops of the netherrack cealing.
Effects: Does 1 HP of damage per second when under it.
Occasionally catches blocks on fire. (Chance increases over time of rain.)
Will set you on fire upon leaving (50% chance.)
Think of it as an accelerated version of the lava drip particle effects.
Details: Moves in a direction much like normal rain. You can avoid it by having a block underneath or being out of the rain.
Lava rain will produce light the same a lava in the areas that it's in.
Visuals: I wasn't kidding when I said it's a faster version of lava drips. That's what it looks like. Netherrack will glow when lava rain is dripping from it. Netherrack will glow when lava rain touches it.
Immunity: Potions of fire restance will let you walk through lava rain unharmed. Holding magma cream halves the damage taken. (1/4 a heart per second.)
Side Note: If they ever introduce horizontal nether portals rain can travel in nether portals.
(What was with all the side notes? I swear either the editor's borkin' or im retarded.)
Another side note: If people (Including mods.) want me to expand on this suggenstion and make it more different I will. There were other threads talking about it but they were either vegue or happening in the overworld.. I can add more ideas if needed.