Perhaps turn häuserkampf in an ability to fortify occupied buildings. Just like setting up a forward HQ, but instead of producing units, offering more protection to units inside and increasing the HP of the building. Effectively turning it into a sort of super-bunker.
Edit: I was wondering if it would even be possible to add a AT-gun that fires from inside the building? Honestly, I doubt it, but I remember seeing a mod that had an 88 in a bunker, so who knows.