Super Smash Brosh for WiiU and 3DS theory
Have you notice that does the WiiFit trainer, Megaman and Rosalina's trailer had in common?
Well if you don't the fighters found those characters and then a battle sequence starts.
I think those are hidden characters and since Sakurai said that they wasn't going to make another SubspaceEmisary, but he was going to do something similar, I think each hidden character would have a video when it's approaching, let's take Rosalina's trailer as an example:
You played an Adventure mode as Kirby, when he ends it the video appears and ends at the part when Rosalina stares, and if you win the battle it will play the part when Lumma is dancing with Kirby.
Summary - "Each Hidden Character would have their own video when it's unlocked"
welp, I said it.