I want the objects in the game to have more use than to just leave them in my house. Example: Pool table. C'mon Ninty, you could throw us a tiny pool mini game in there. Same with slots, weight benches, etc. There's an endless array of objects to interact with beyond the simple placement mechanic.
Additionally, give us more ways to orient stuff in our house. I hated how you couldn't really organize anything. Let me use both shelves of a book shelf, let me put things under the table, etc. Just more inhouse customization.
Beyond that, competitions with other players, online stat tracking (if they did allow competitions/games, etc), maybe eventually connecting multiple cities together to open up shops (more MMORPG-like, but not too much). Perhaps some central hubs to connect to where Ninty has set up special operations/missions/etc. Make me want to log in because there's a competition and I can win a super rare...something. I dunno.
AC has huge potential, just needs people to back it up.
Somewhat offtopic, with the Rev, let people homebrew stuff for AC and put it online. There's mods for endless games on computers, and I gaurantee you people WILL take the time to make something for a game if they have the tools and enough people beg for it. Now THAT would be awesome.