Clouds and weather would be awesome. Might be a ways off though, I'd imagine quite a bit would need to be overhauled to get that feeling you're flying through rain/snow and to then see it visualy represented on your canopy (much like how when you recieve an oil hit you have oil streaking down your canopy).
It would however be really cool since we could then maybe have a beter visual of water/oil streaking over your canopy (maybe even have it worked into the damage model, the left side of the engine got shot up, so oil comes spewing out of the left side and all over the left side of the canopy). As it is now, I think someone flung a giant water ballon filled with engine oil at my canopy whenever my oil takes a hit. This would be a huge overhaul of some parts of the game, but worth it imo.