Since end game in story mode comes quite quickly, one ends up buying and flipping the same houses over an over, while keeping the really nice ones as "trophies". As it is not possible to keep a copy of your work, you can either keep the house or sell it. So.. having more big houses that involve far more work would be a nice addition for the collection. Think about that gorgeous mansion (or villa) in Uncharted 4... Ideally, my idea is to make each of the houses a true marvel and then probably proceed to the sandbox mode. That would be the story mode completed for me.

P.S. In general more houses = more creative opportunities = more fun!