Avatars can be PC's and NPC's. Dress the NPC's in regional clothing of player's choosing. Make them semi-intelligent and can assign jobs to them. Can assign behaviour to them based on your rank. EG, if you are a king they bow, if you are a president they cheer, etc.
Abandonned buildings deteriate and make ruins.
Trees, crops, etc, based on season and land they are on. Much more to it, but that was a basic thing. I have been obsessed with it for years, long before we used computers. Used to do it in clay. Don't know if from what I've read this is what you are thinking, but if you are reading this, I assume you are interested in something similar.