I remember transporting down to DS9 for the first time. The awesomeness of being able to walk about the familiar starbase ... and then discovering ugly ramps, and made up rooms was disappointing. I'd love a redo of DS9 to include the habitat ring, with a comfortable hotel feeling, and personal quarters. Especially now that much larger environments are capable (eg New Romulas).
Yea, I wouldn't mind getting a canon Promenade, either. Quarters on DS9 might have been nice before Season 7, when you spent a load of time there for STFs, but now it's a ghost town. Not as much point there anymore. Would still be nice, though. Or maybe on Bajor, in Hathan.
I wouldn't mind taking up a residence on Mol'Rihan, however, if you earned it (a Reputation reward, maybe Tier 5, and with something extra for having it there).
Actually, come to think of it, you could have available plots on almost every Social Zone world. That way, RP'ers playing Vulcans or Andorians can have a place on their homeworlds, and I know a FEW people that would live on Risa.
Also, it'd make sense to max at about 3 (Fleet Base, Ship, and 1 Social Location). However, at an EC cost, you could still move your Social Quarters to a different one (EC amount would vary depending on customization level and distance between old and new - Someone's gotta pay the freighter captains).
But I wouldn't call interiors functional to make me want to go back. I never visit my interior because... well its boring. They were never really finished.
There are some unique DOff missions. Tribble gene splicing, counseling sessions, Culinary Credentials. Nothing that gets anyone off. Usually, I stick to my Department Heads until I can't do anything anymore. But you are right, interiors really have no use. They're not done. And the "Captain's Quarters" on-board are bare-bones, bland, and totally useless. Better to implement a system for it.
Doesn't Champions have Hideouts? All the STO team has to do is request the code branch and translate it. Then again, CO also has vehicles, but we don't have those yet, either.