I would definitely love to get more stuff to sink money into, even if it's just highly priced flavor/filler like the spa room thing you can set up, which I think is decent as a frivolous NPC-designing indulgence that doesn't even make back any money IIRC (unless the desk person does? maybe?). Time wasting household mechanics fluff as reward for puttering around is catnip to me, I happily lose whole evenings in games with that shit when I'm in the mood for it, but LT's minigame runs out of goals in a hurry once you've got the baseline of getting all your NPCs equipped and customized how you want.
I don't know what other fancy rooms you could put in the mansion, I'm too tired to theorycraft, but I think that's probably the easiest path since it's not like you could house NPCs in all those rooms anyway without blowing up your save. May as well find some other way to flavor up that space.
In the past I think I've seen people suggest being able to buy more properties in other locations, which sounds cool in theory, but it'd probably need to reward something other than moneymaking as that'd only accelerate the problem.
