Ive noticed throughout my time playing Cataclysm, that i will often rush out of doors and right into the rain because i forget to look at the weather display. While this is my fault and is probably a behavior that can be changed, recently i watched a Cataclysm video which had the player character hallucinating. One of those hallucinations made it look a lot like it was raining, i think it was putting a handful of either white or blue-ish vertical | (pipes) into the terrain, but now i cant seem to find that video to exactly recall the exact effect and link it for example purposes. Could make for a nice method of simulating rain or potentially snow at some point. The color and density of the rain could indicate what kind of rain or how heavy it was falling.
There would obviously be some problems to consider, the effect would need to take into account whether the tile was considered outdoors or not. One would need to consider whether or not rain drops would obscure items that were on the ground which wouldnt be a problem if you were moving, but might be if you were standing still. Would also need to consider whether the rain would have a performance impact on things that pass multiple turns at a time, IE: Is it simulated, or is it simply a graphical overlay that updates whenever a world update is called for.
Ultimately it would probably mainly be a cosmetic change, and thus a relatively unnecessary addon to make considering all of the more important things out there that need doing, but if someone should feel inclined, might add in an extra element of atmosphere to see the weather happening outside as you pass turns inside waiting, or while you are moving around. Would give the player quicker feedback about the weather when they were spending the majority of their concentration on the overworld display.