Not speculation, but an idea my brother and I just had.

[LilithLissandra]

The Waluigi Smash reveal trailer: He's walking down a sidewalk with a mail bag slung over his shoulder. You get a short montage of him delivering letters to each fighter in turn: Joker, Piranha Plant, etc. He sits down on a bench, wiping sweat from his brow, and as he does he notices one last letter in the bag. He reaches in, pulls it out to check who he forgot to deliver to, and it's revealed that the final letter is addressed to none other than himself. He drops the bag and leaps to his feet, having a brief moment of complete joy, finally having received that coveted invitation. And as he's about to run off, he looks back at the mail bag, already looking abandoned. He looks at the letter, then at the bag, and makes up his mind. He slings the bag over his shoulder and walks off scene with a look of determination in his eyes. Cut to the splash intro...

Waluigi, the official Smash Ultimate postman, has joined the fight.