Given the fact that most recent newcomers use purely the smash fighters in their trailers, this would be my take:
Marth, Link, Bowser, and Ganondorf are fighting on a generic seeming field. Ganondorf and Bowser are defeating the characters. Then the screen turns into what it looks like for Shulk's visions, turning white and giving us a glimpse into Link's defeat. Link is barely standing, as Ganondorf uses his sword (because he has been decloned with some sword moves) and uses an over head strike, to only see his blade stop by a familiar red blade. The swordsman pushes back against Ganondorf as he shouts "The Future doesn't belong to you!", and we get an insert of Shulk with his sword:
Shulk Sees the Future!
This music is used, with a faster tempo:
http://www.youtube.com/watch?v=-NLrE2R-5eo
Then we go actual gameplay footage. We get a good view of the Bionis Leg stage. Shulk uses all his key moves on characters like Mario, Bowser, Link, Samus, Rosalina, the VIllager, Mega Man, Pikachu, and Marth. The moves we see are Cyclone, Buster, Speed, Purge, Back Slash, Shaker Edge, Stream Edge, with him saying quotes like
"Streeeam Edge!" "You're not invincible!" and "Not good enough!" and it ends with him saying "Behold the power of the Monado!", before we see his actual final smash.
Then we go back to the original battle. Shulk helps Link up as they prepare for the fight. Ganondorf and Bowser turn into Ganon and Giga Bowser respectively. We get the trio of swordsmen all readying their swords, with Shulk in the middle. Both Marth and Link look at Shulk, as Shulk closes his eyes for a vision. The camera zooms in and then goes to white. We get the SSB4 logo.