I think a villain trailer is the most realistic. A lot of important villain chars are missing.
A trailer could be revealed on the gamescom. There are only a few weeks away to the release in japan, so the hype must be raised.
I think Wario, Metaknight, Ganondorf and as a newcomer King K. Rool could be revealed.
One week before the release, would come the last trailer in a Nintendo Direct, which takes the hype to maximum.
This trailer should revealed Ness, Mewtwo, Bomberman and as last Ridley. I think lot of people would not handle this hype anymore :D
Here my new 52er roster. Right are 2 DLC Packs, one Newcomer Pack and an another one which ist called Veteran Pack.
I dont think that both could made it. Only Fire Emblem had become 2 new characters and now we dont have a lot of spots left