As a die-hard Smash fan, I'm super interested to see where they take the series next. I'm of the mind that Smash 4 is pretty much the best of the series in terms of marrying the design sensibilities of both Melee and Brawl (well, as much as they can be), so I certainly wouldn't be opposed to them just using it as a foundation for the next game. It plays well, feels fast but not overwhelmingly so compared to its GameCube brother, has less of an overt focus on the defensive game than it's Wii counterpart, and just runs smoothly and looks gorgeous. Obviously, I could nitpick and there are a few things I'd like to see changed, but overall, I think it's an amazing achievement -- hopefully they don't throw everything out and start completely from scratch.
In the end, I'm just down for another fun game that glorifies and celebrates Nintendo's rich and weird history. Add some new characters, some light gameplay tweaks and balances, new stages (and bring back some more old fan favorites please!) and cool new modes and I'm in. Of course, I'm basically "in" regardless so whatever.
Just, y'know, make Wii Fit Trainer better, Nintendo. She's the best.