I always enjoyed these in campaign mode, but they wouldn’t make as much sense here. Still, thought it’d be fun to suggest some, and if they’d even work out.
COs like Karax, Swann, and Nova would probably just outright destroy the building with their top bars (Orbital Strike, Laser Drill, and Griffin Airstrike respectively), but it could still be fun to miniaturize its effects for the sake of of being inside (I mean, a CC Sc1 is supposed to be 3x4 building space in a regular match, but the interior mission has its size at 128x128 in terms of map size). You could still have Ultras and Brutalisks inside, even though they’re supposed to be the sizes of buildings themselves.
Otherwise, you could have reinforcements just materalize like normal, or force them to come through the front door or “new entrances”.