You have no idea how happy aqua regia kingswater makes me. Also! What about a liquid that attacks anything near it and occasionally drags itself out and tries to find more liquid to corrupt. It might also take on some characteristics of whatever it corrupts, exempli grata: if it corrupts lava things get hurt just by stepping in it, or anti grav liquid makes it float a little bit.
Oh yeah! Hail from hell baby! Crystal Rain that could be summoned at will would be an awesome Glunaught Planetary Defense... *Notes to self*
Other liquids would be awesome to like if you can melt gold to golden water and diamond to diamond water. Acid rain would be cool to.
I'd personally like to see planets that precipitate ash and rocks. You'd probably want to build covered walkways for that kind of stuff.
Perhaps the various rains could have different states of matter? I'm just saying, what if you get a planet that's extremely cold but has acid for rain? Starbounds whole premise is planetary uniqueness, so this is totally not even out of the picture. Acid snow? Yes please!
It might be time consuming to define how every single liquid interacts with other ones :/ Although it'd be cool to see how they mixed together, and different densities could lead to cool stuff. What if all/most liquids were dye-able? You could make such pretty acid pits. It would be delightfully murderous.
I think that on irradiated planets or biomes that the rain/snow should be black and be, you know, irradiated.