That's a lot more work than a cosmetic change, and at this point the developers seem ready to feature-lock.
Since work has been started on day & night schedules already, I'm assuming the passage of time and appropriate changes to light sources are already in place (and that controls for this are built into the editor to begin with). The same goes for weather. I don't think we can expect to receive anything beyond that cosmetic-only change, however: it gets dark, it gets light, it gets overcast and rains, etc. And, honestly, I have strong doubts even that would happen at this stage.
Without "schedules".. without NPCs of all types reacting to the changes in both time and weather, this particular system is going to be a pale shadow of what it could have been. But that's completely off the table at this point, and just seeing the game world (which is very nice looking graphically) in different stages of lighting based on the weather and time of day would be a treat.
For example, look at these two scenes:
Now, they're not the same location, but you can imagine they could be, just at different times of day. Even without schedules, I think it would be neat to see the lighting changing the scenery over time like this. It keeps things visually interesting and helps prevent treks across the same piece of landscape from getting stale over time.