I'd really like to see something akin to how some tycoon games work - like Prison Architect. If you've never played the game, you plot off specific areas to be, say a dining hall, or a cell, or visitor's room. Then that area is required to have the things to actually make that area function as intended.
So for PC, you could map off a 4x6 plot as a restuarunt, then it will require having a kitchen / whatever, at least 4 tables, etc. Then you could map another area off as a bathroom, which will require toilets, sink, stalls.
This system would be something I think that would really make buildings almost completely customizable.