I had forgotten about the whole Keep thing in NWN2. That could have made a great foundation for a game unto itself. You start a game with a crappy ruined keep and you want to upgrade it. You also have a group of adventures who recover gold and artifacts (which you actually play the adventures, and they are not just some arbitrary unit). You level up your adventurers and you keep which provides better goods / services to the adventurers. Maybe you take it to a kingdom level too.
Are there any games like that out?