I think they need to push the series into something that more resembles a virtual world like "second life" than to continue down the road with only "local" animals. There were some significant improvements in the depth of gameplay from the GCN to the DS, but the leap to the Wii needs to be monumental. More towns to visit (that are online (owned by others) and offline (randomly generated at town creation) and part of the overall "local experience"
More items overall like the fish, bugs, bones, etc. for the museums need to be significantly deeper.
The "online" mode needs to have a bulletproof way to allow people to visit and not damage your town and/or mess up your local play. The DS has too many exploits to have people you don't know in your town.
Animal "Miis" need to become a part of the game and it needs to be deep in terms of customization.
Multiplayer in the same town -- i.e. multiple people living in the same house or town -- even if they are not local players -- needs to somehow be accomodated. I.e. if you and a friend or three want to share a town or large house, there needs to be a way to allow everyone to exist and use the one house and keep inventory, actions, etc. all on one machine with the ability to "transfer ownership" to another machine should one person want to opt out. If one could leave their Wii on and connected, that would be rather cool...
Just a few ideas, but the series really needs to be expanded as another game that is a limited expansion of the DS title will really disappoint....
I hate trolls.

Systems I currently own: 360, PS3, Wii, DS Lite (2)
Systems I've owned: PS2, PS1, Dreamcast, Saturn, 3DO, Genesis, Gamecube, N64, SNES, NES, GBA, GB, C64, Amiga, Atari 2600 and 5200, Sega Game Gear, Vectrex, Intellivision, Pong. Yes, Pong.