Well if its raining too much no one will play!
I like this idea though if we are talking about a wish list.
Some Fog limiting visibilty would be realistic and interesting.
Light rain or drizzle would really be great because it would affect ball travel adding extra challenge!
Not sure what else but I added this to the ever growing wishlist here: http://wgt.com/forums/t/8674.aspx
Its a very ambitious request but hey, you miss 100% of the shots you leave shot, we have to at least try! *-)