Basically where you play the Baron (or Count/Duke/King/Emperor) and their family like in the Sims and build their castle. That's a game I'd play the heck out of and the only way I can think of to make me want to play a Baron.
Yes, there is an old, old game called "Sims Medieval" but that didn't have families and it didn't allow you to build a castle. It was pretty much a whole game centered around those fetch-quests in WoW.
The problem with Sims Medieval was that it wasn't actually a sims game in a Medieval setting. I would have played that to death.