There are a couple things I dont like, either... But Ill deal with them lol. Like skaw said, it would be nice to share a town, the only problem with that would be that your town would need to be hosted on a server somewhere. With Wiiconnect24, it wouldnt work so well, considering some peoples net gets shut down or disconnects every so often. Updating would be a little hard to keep up with if hosted from the users consoles.
I dont like the Wii Speak thing... I would much rather have a headset, personally. I imagine a headset will come sooner or later, though. Theres a couple different methods nintendo could use to enable them.
Online towns would be cool, too. This is sort of like what Skaw mentioned, but there would be several hosted servers, and on each server there would be several towns, and in each town a certain number of people could live there. From this point, you could transfer to different towns on your server (this would be like going to a different town or state in your own country.) This doesnt offer much, other than different layouts of towns and a small difference in item availability.
Besides visiting different towns via the train, you could visit an airport to switch servers (which basically acts as different countries) and from those servers select another town to visit. This would be nifty, because you will be enveloped in a completely different place (different scenery, vegetation, cultures, etc.)
Thats what I personally wouldve liked.
Edit: Also, to allow members to visit any town/country they would like, it would be a good idea to put a time limit on how long you could stay in a certain town. Such as, if it isnt your native town, you have a 60 minute time limit. If the time runs out before you leave the town, that towns police come and take you to the airport by force and ship you to the town selection screen (on the current server.)
To join a town, you could just select it, and you would be put into a queue. If the queue is too long, you can try a different town.