I love the idea of buildings ranging from shacks to sprawling castles, but perhaps you could include more than one choice at a particular level? For instance, as a wizard I'd prefer a tower full of monsters and golems over a mansion, and Wizard's Guild might prefer some sort of academy-themed building over a keep.
Different "houses" at a particular level might have different strengths and weaknesses, i.e. a keep has the strongest defensive abilities for its level and generates minions at a set rate (up to a certain number), an academy has labs and resources for research and item creation, a guild hall has trainers or guild-only vendors that save members money, etc.
Different "houses" at a particular level might have different strengths and weaknesses, i.e. a keep has the strongest defensive abilities for its level and generates minions at a set rate (up to a certain number), an academy has labs and resources for research and item creation, a guild hall has trainers or guild-only vendors that save members money, etc.