im not sure if these are already added or have been suggested but my ideas are:
1. shadows for clouds
2. clouds affect grass growing
3. rain, which speeds grass growing, cube would be covered with a rain droplet pic.
4. changing speed (like having a gale)
5. come at different altitudes.
6. different types.
How is this realistic? Grass doesn't die in real life on a cloudy day. It's already unrealistic enough that it dies in shady areas in game. Imagine moving patches of dead grass lol.
I would poop myself if dead grass was following me.
On a serious note, I think theres a bit more optimizations fixes to be done before this update. Although im using a gaming rig, weather effects could POSSIBLY be taxing on older machines, where Minecraft seems to do well on.