Here's how I think the trailer will go:
We open up with the standard Smash flame opening before going to Battlefield. This theme starts playing:
One by one, we see Mario, Sonic, Mega Man, and Pac-Man enter and square off. Then the logo pops up on the screen. We get a montage of different battle scenes, starting off with the four in Battlefield, then cutting to Link, Kirby, and Pikachu fighting on the Halberd when Meta Knight swoops in and joins the battle. There's a few seconds of Meta Knight facing other characters and showing his moves. Then we cut to Meta Knight being ganged up on by Pit and Zero Suit Samus at Palutena's Temple. Then Wario rides in on his bike, crashing into the others and sending them flying. We then get a montage of him beating up other characters before Wii Fit Trainer sends him flying, crashing into a robot.
The music comes to an abrupt end as Wario falls to the ground and gets back up. The robot starts to advance on him as Meta Knight and Ike appear to help. Then some Xenoblade music comes on as we see the robot getting attacked. Upon zooming in, we see that it's Shulk!
Shulk Smashes into Battle! (Or something.)
Then we get Shulk fighting against the different characters before the logo pops up. Then the bonus scene starts, showing a cardboard box in the middle of an arena. Shulk walks by and stops to look at it. After a while, he smiles and continues walking on his way. Then we fade to black...