Hopefully they will make minor plug ins, with new weapons, seasonal foliage, adoption, remarriage, divorce, new houses. I bet if they were to release seasonal foliage it would be in the fall. Since it would be something minor they could easily release it by then, and I bet it would be for five dollars.
I don't know if the seasonal foliage is such a good idea.
Consider:
If the trees and flowers and such start changing according to a seasonal schedule, you have to expect other changes to accompany them - for instance, snowfall, rain, streams swollen and flooding from snowmelt, significant landscape changes (for instance, various passes getting clogged with snow after storms, or even avalanches), and that sort of thing.
The insects and animals would be affected, too.
Be careful what you wish for, because the changes could wind up being quite profound, and perhaps even discomfiting.