I think buildings are underused so far in the game. How about different buildings with certain themes that teach different schools of magic only in that building? Would be pretty cool with a mod that has castles and such that actually has a meaningful function other than aesthetics.
It would also be cool to have research decided by the villagers themselves. Like these schools of different types of magic could battle it out against each other. Say every “year” (like every 20 in-game days or so?) you could have some kind of tournament where the winning side decides what type of magic will be focused for the coming year. That could in turn decide what theme of research that will be unlockable. This way each new village you play could become very different. Sometimes you might do eastern magic, other times norse, other times dark etc. Maybe you could have effects like the weather change for that year to match the ruling school of magic. Maybe even the moods of the villagers change? Each school of magic could have strengths and weaknesses. Like with dark magic winning the tournament, all villagers would have potential to become very powerful blood mages, but it also comes with problems like crime and low morale.
You could perhaps also alter the world around the village depending on this. Maybe you can choose what your first villagers will be (good/evil/whatever) and let this originally affect what magic will be of interest. But then depending on quest-success/food quality/happiness you could “inspire” the villagers to go different routes without the player being too much in control. All this could add a lot to the RPG feel of the mod while still leaving a big part of the village simulation intact. Maybe you could have a single villager as your main character who is the only one you can control, while all the others live lives that realistically decides who they will become.
Then i guess another big part of the mod would have to focus on the quests and storyline. I guess you’d just have to be creative here. Some quests could be underground. The goals could differ between killing, fetching, making, talking, charming, curing etc.
Oh, and you could have powerful mixes of schools of magic create new combos and such to give the player a reason to actually build all of the school buildings. Otherwise the player would obviously decide the fate of the village entirely by which buildings they place. Maybe you could even add procreation into the mod so that if two mages of different masteries create offspring, that person will have unique abilities. Maybe you can also get to choose companions to have with you on your quests.