well an idea that springs to mind when i see this is sort of a fable-themed real estate agency. players could buy properties already in the world, update(upgrade) them and sell them or rent them for a price. if they sell then it would be to the players possible profit, if they rent it would be a portion of what the price would be if they sell every 3 days or so.
Not bad, but it has a few problems that need addressed, one of which fable had too. Renting a house out would just generate cash indefinitely, leading to a huge imbalance. No need to buy and sell when all you do is dump money into a few places, then take that money to do it again. It makes game play awful flat, and if anything would actually undermine the purpose of the guild's creation.
As a MW problem, making pre-existing spaces upgradable would lead to many conflicts with other mods, especially if the homes are scattered across the map. To do it with less conflicts, you'd have to create an abandoned town you bring to life, or make an interior cell that appears like it's an exterior. It might be a neat idea to have a town that over time reflects your influence and choices.. maybe it even becomes the new head quarters for the guild; so the idea has merit, but only if it could be completed.
In fact, if the town were such that as you sell off houses, or help someone get a book store etc, and if you see different stock than if you made a different choice, and say you can people start moving in to the buildings you sell.. if it were like that, it would have it's own visual reward, and even some replay ability.