I'm kind of curious to see how much they could add on top of the existing formula without turning it into a completely different game. I think the ideal would be to have a persistent world, but not really an MMO, where you can share a town with a few friends, each with a house, and each able to access it at any time, regardless of who was or wasn't online. This could easily be done, although it would require real servers or they could take the more complicated route of a really sophisticated back-end that made it look persistent for everyone but was really just save file swaps and peer-to-peer multiplayer.
Regardless, just like I buy Madden (and NHL and FIFA) every year, I'll continue to buy Animal Crossing even if it stays with the iterative updates, because every couple years I get the itch to play it again and I might as well play the best version available.