To me, rain, storms and to an extent fog are the "basic weather" I'd consider in The Sims. These are really independent from seasons changing but since they are a part of it it makes sense to patch it in to complement a future seasons expansion. They just need to add umbrellas, a reaction to the rain by sims and gray sky instead of blue, maybe a new type of cloud that is darker and thunder fx for storms. The worlds landscapes don't need to be changed, we already have ground puddles and the rain could just automatically water all plants. Of course I don't know how any of this really works in game development.
