A texture sheet large enough to have 4 material versions for each material type (4 grass, 4 stone etc…)
An active day counter.
As you reach day 90, 180, 270, and 360 color shifts, lighting shifts, and weather shifts occur.
There could also be other elements that rely on the day counter.
- Plants growth differ in different weathers and seasons.
- Animal patterns, and types of animals present differ during certain seasons and weather types.
- Running electricity and food quality could be tied to the day counter aswell… This might allow for a hardcore mode where electronics and lights get disabled after 30 days, certain foods start to rot. (project zomboid does something like this very well)
I’ll try to edit this post with more ideas in the future 
Everyone feel free to give me your thoughts on these ideas or add new ones!