Yes, it would be nice to be able to add on some additional rooms - apart from what is already there in the structure.
I would like to put in the sauna, but there would have been nowhere to put a family room with pool table, play kitchen, bookshelves, couches, TV, etc. The living room (room off of front door)is too small for that, and nobody wants to enter someone's house in the informal setting of a den, where people just hang out.
It would be neat if on the back deck, we had an option of building a little sauna room.
Also, it would be really nice if they would allow us to move that big tent so we could have more room in the back yard! A lot of players have commented on that, but nothing happens.