I just 100%'d Smash Wii U a few weeks ago (As far as Challenges go, anyway) so as much as I love Smash to the point of it being my favorite gaming series, I wouldn't get it at launch if it was just a port of Smash Wii U with all the DLC.
However, thanks to the existence of the 3DS version, there's a lot of content they could add to a definitive remaster. The 3DS version has almost 30 exclusive stages; it also has its own challenges, trophies and Smash Run, which, for my money, is infinitely better than Smash Tour and would benefit. HDify all of that, make it so the DLC characters/franchises are more integrated in the game (giving them their own special moves, giving them their own Events and Challenges, give more music and trophies to the likes of Street Fighter and Final Fantasy), and voilá, you have a hell of a package, the ultimate version of Smash For.
Mind you, I firmly believe Sakurai's involved in this, as much of a vacation as he claims to have gotten, and wouldn't be surprised in the slightest if he can't help adding extra bells and whistles like new stages or even characters. At the very least, Ice Climbers and Wolf I could see back on the game.