When terraforming and leveling land for your village, castle, etc. the inside is usually just a bunch of tilled dirt -- barely befitting of a prestigious settlement. While tilled soil has its place in the field, it would be nice that settlements, once settled, don't constantly look like construction zones. Moreover, just as land claims deteriorate and trees slowly regrow and spread saplings, it'd be nice if grass would retake fallow fields that haven't been separated well enough from native grass (similar to how Minecraft's mechanic works with grass spreading slowly to neighboring tiles but is mitigated if there's something like a fence or path in between). Not only would it make settlements look greener and happier, it'll also add to the liveliness of the world, retaking old settlements and fields that were once inhabited and now lay abandoned, waiting to either be reclaimed by nature or rediscovered by passersby. If you wanted to get super ambitious, vines and ivy could also retake areas that are seldom traveled.. would be really neat finding old ruins which would lend to ever-evolving player lore.
In terms of ground type conversion, it'd be also nice that there were easier ways to spread forest tiles or turn once-forested land into plowed farm fields. Right now, you have to dump a layer of whatever soil in order to convert that manually, but if we're going for hardcore/realism, nature should play a role in terraforming as well over time when players haven't touched it and interfered themselves. As trees spread, so too should the undergrowth and "forest tiles" underneath; whereas players might be chipping away south of a great forest, it might have spread slowly in other directions, changing the map as well. It also shouldn't really matter where one plants trees (on forest or fertile tiles) as the dirt's good either way, though obviously crops would need to be on fertile, tilled land. That being said, it'd be nice to have some "wilder" options when it comes to early agriculture such as planting actual berry bushes or finding wild grapes or patches of onions, corn, beans, peas, etc. instead of just "gathering wild plants" (which can still be an option, but finding those patches could be a godsend to the starving nomad).
Both suggestions would make the world feel more alive and vibrant and give more PvE options to players. These mechanics can be slowed or sped up depending on a few other factors (being within claims or spreading grass seed manually, respectively), but if they naturally occured, it'd make the lands less static and artificial-feeling. Plus like I said, it'd be really cool if you were a player either returning from a hiatus or a brand new later-game player and stumbling upon old ruins of settlements or noticed that the map has changed quite a bit since you were last around, keeping up that feeling of awe and discovery rather than knowing where every other settlement is.