With the popularity of the tropical island setting and the different mini-games you can play there, I think it could be neat to have a snowy lodge and ski resort to the north of the map. You can travel up to the mountain via a ski lift, and there'd be tons of snowy themed mini-games, furniture and prizes. Maybe like the original AC on Gamecube, you can own your own cabin and decorate it.
And maybe it's time for a bit of a bold change... with the graphical capabilities of the Wii U, maybe it's time to update the log rolling, top-downish angle to a more third person, three dimensional perspective.
And rugs! I want to put rugs and carpets around on my barren wooden floors, wherever I want!