They have to do something to make this more compelling than just a GoTY edition, especially since the game is available on the 3DS in addition to the Wii U. If this rumor is true, I'm expecting at least a couple new characters, even if they're Wolf and Ice Climbers. I think that that would be much more likely to drive sales of the new console than a slight resolution upgrade or something. Smash 4 already runs fantastically on Wii U.
Super Smash Bros: The Masahiro Sakurai Collection
-64, Melee, and Brawl bumped up to HD. Online added. Cross-gameplay playlists.
-New version of 4 with all stages from both versions, plus all DLC, plus every past character and past stage added in. Chrom, Chorus Kids, Ridley and K Rool added.
Included as a pack-in for the NX, which retails for $199.99 and is out tomorrow, alongside Mario Kart 9, Super Mario Galaxy 3, Pokemon Sun & Moon, and Zelda NX.
Nintendo declared bankrupt about a week later.
In reality, I think it'll be one of the following:
-Port of Melee in HD with online added
-Port of Smash 4 Wii U with DLC included. Will add some of the following things: 3DS stages, non-returning stages, cut veterans, characters that were likely in development to some degree but cut (Chrom & Chorus Kids). Will almost certainly not add all of these things, especially if its out in time for launch and Sakurai isn't involved.