I was hesitant to suggest this before, but I now realize there is a lot of tiny exterior homes with large interiors, so...
Interior design: Allow us to use large amount of in-game money (5-10m) to replace our house interior with another one that is nicer.
Why?
I have a lot of money and I want to use it on something refreshing/new. I'm down to pay 5-10M for redesigning my house interior to something I like. There is many interiors in-game, but some of them are limited to a few properties and unavailable to most people.
Why not buy an existing house with that interior?
Mainly because they are located in a place I do not like, which makes spending my money on them unsatisfying. I rather have a house that means something to me redesigned.