Animal Crossing has been a very consistent title from Nintendo. Every new game adds several changes which keeps the rather simple gameplay elegant and charmful.

At the same time, the most powerful console the IP has been on is the Wii, which isn't exactly a powerhouse. With the Switch coming, I think Nintendo will probably be capable of doing things they weren't able to before.

Personally, I think anything they make will be fine by me. However, I'd love to see a pet feature be installed. Your pet would have a small house next to yours, and you can take it to walk with you by carrying a leash. Chances are Nintendo won't do this, but it'd be a very adorable addition to the series. 

That aside, I'd like to see everyone else's ideas.

MMO lite. I also like your idea about pets.

Pets sound interesting, but I guess it would feel a bit weird since many of the characters are animals themselves.

I like your idea but it should involve the animals themselves, maybe like a roomate thing.

There should be a debit/credit card thing for big money transactions. You should also be able to send other players furniture or gifts somehow.

Beastiality relationships with the other villagers and between them, with pregnancy and children. I want to see what the child of a human and an anthropomorphic duck would look like. Or a wolf cat.

I would like more personalisation of the town and the house.

More ways to earn money.

Have some online event where you see other players.

More events in general, and I hope the whole mayor thing continues. It would also be amazing if the animals in the village actually helped on the community projects (by more than the 5 bells a year they do in the current games).

Also, I need grass seeds. And more storage in the house please :/

Take all of the spinoffs they have done since New Leaf, and put them in the new game. Simple as that.

Underground tunnels, Extensive character customisation and splitscreen co-op.

Yeah, this. Would kind of feel like slavery more than anything else...