Have parcels of land avaiable to buy at all cities.
Build your home based on Tier of resources.
Have missions to collect permission and a contract to build in a town.
Pick your design from presets
Customise colours
Modular parts like cellars, chest rooms, trophy rooms
Make it upgradable past the standard tier 4 but price materials accordingly
Make all building materials Bind on pick up so its fair game for all
So much Potential.