sakurai said he wasn't going to because the cutscenes for the brawl one got leaked
He could just do something cutscene-less like Smash Run was. How about entire areas based on a game, where a character has to go from point A to point B, and fight a boss at the end? Mega Man, for example, could go through various stages and fight robot masters at the end, and Pit could fight underworld enemies to get to the end and fight bosses like Pandora, Dark Pit [Unlockable], and [SPOLERS:
Hades].
The Wii U probably has plenty of hardware space to support something like this for all characters, although I wonder what Wii Fit trainer and the Villager would have for this...