I think it'd be cool if they centered the next original Smash Bros. entry around Sakurai's own take on the tagging mechanic we see in some fighting games (they could literally call it... "Smash Bros. Switch"). It would allow them to keep the core mechanics of the series intact, at least in some fashion, but it might also give Sakurai room to add enough new mechanics and gameplay changes that it feels like a truly significant change to the way Smash Bros. is played. It would also fall in line with the frenetic, ridiculous action the series is known for. Bring some Project X Zone staffers on board to write the dialogue (or, better yet, make a full tactical RPG/Smash Bros. crossover story mode) for maximum random crazy bullshit.
And, yeah, I'd also like to see the next Animal Crossing entry move towards a bigger town. I enjoyed the core mechanics of New Leaf well enough, but the size of the town kind of took me out of the game. I don't think it has to be gigantic, as that might take some of the sense of community out of it, but I would like to see it expanded.