I know most people seem divided on the "brand new game" or "Wii U port" split but what if it was something in the middle? Like, take the Wii U game as a base, but add enough significant new stuff for it to be considered a new game?
That would be the ideal situation for me. I think SSB4 is almost perfect mechanics-wise and I love the sheer amount of characters, stages, and content in general that it has. To throw all of that out of the window seems like a waste, IMO, especially considering the best version of the game was on Wii U, which really limited its audience.
They can really go all-out with this game if they put everything from the Wii U and 3DS games (including DLC), plus some newcomers and returning veterans, give it a new coat of paint (new title, menus, main theme, etc.) and add some significant new stuff. And by that I mean a new story mode like Subspace Emissary (not gonna happen but let me dream :P).