I thought up another possible reveal. For some reason, a big thing among some YouTubers is that "Waluigi isn't actually an Assist, he was knocked off screen and Sakurai is just tricking us" which I don't believe. Let's blue sky it for a moment.
We see the footage from E3 of Marth knocking Waluigi offscreen. Suddenly the quality turns to that of a reveal trailer and we see Waluigi falling hard on his rear, with his tennis racket in hand. He stands up and hesitates. He looks around, clearly confused that he hasn't disappeared yet. He adjusts his cap and leaps towards Marth/the camera. "WALUIGI assists himself to battle!"
While running towards Marth, multiple Assist Trophies appear. Waluigi picks one up and tosses it at Marth's head, making it shatter. A trembling, angry-looking figure stands up and points at Waluigi. The camera zooms in and it's a furious-looking Ashley. "ASHLEY casts a spell!" and she attacks Waluigi with magic from her wand. Gameplay begins, showing both characters using Assist Trophies and popping out character after character.
The video ends with Waluigi and Marth running away from a furious Ashley who is holding an empty Assist Trophy, implying she'll shove them both inside of it.