D.T. said:
1. A police station. Maybe it has Copper and Booker and the gate has new guards, maybe the police station has new officers, whatever. Not entirely sure what the functions should be yet but it could have the villager report function instead of the Town Hall. Another idea...Nintendo could actually start moderating online encounters, like if someone screws up someone else's town with hacking you can report them through the police station. I think Microsoft moderates hackers online so Nintendo should be able to do it too.
2. An arcade?
3. Maybe a building in the city that you can rent if you want, as an extra place for you to put your stuff. Like an extra room, sort of like the island house in ACGCN.
4. A store devoted entirely to plants and gardening. They could also expand on the tree types and gardening and stuff so the store has more of a purpose but I don't have any ideas on that right now.
5. Perhaps the official Farway museum, the one the branch in your town is always talking about.
6. A gym, where you can do aerobics like you could in the GCN game, and other stuff...