There are a few visual changes I'd love to see:
1. Snow depth. Obviously. But I've given up hope on it after they wouldn't even do snow depth in the Snowy Escape EP which was all about snow. D:
2. Spring needs an overhaul. It's one thing that every The Sims Seasons has missed. In the Spring, trees blossom before they grow leaves. If this was implemented in The Sims, it would make Spring so much more vibrant and fun visually.
3. Enhance the rain effects. Visible raindrops on the ground (we have this in the water, so why not everywhere else?), rain drops off of the roof (like in The Sims 3) and perhaps even other places (since IRL rain drops off of trees and lots of other things). Also I think that surfaces should become reflective. This is a really pretty effect that is used in a lot of games.