Alright I guess I might as well attempt to write my own bout of fan fiction.
Job 91: Wishing Stars
The trailer opens up on Final Destination with Master Hand overlooking the several worlds of Smash Bros. through different portals. The camera pans to the different portals showcasing what each character is doing in their respective worlds. Kirby is fighting off King Dedede who once again appears to be hoarding all of Dream Land's delicious food. Dedede swings a Waddle Doo at Kirby his hammer. Kirby inhales it becoming Beam Kirby and fires off an electric beam from his want at King Dedede sending him flying.
In the Mario world, Mario is battling Bowser Jr. atop an airship. Bowser Jr. shoots a cannon ball from his clown car that Mario narrowly avoids in slow motion. In the Zelda world Link is surrounded by Bokoblins as he desperately attempts to fend them off. The portals show off the various worlds faster as it goes by each one, passing by Donkey Kong and Diddy Kong swinging through vines, Samus getting ready to fight Mother Brain, Captain Falcon racing and Pokemon Trainer commanding Charizard to attack an opposing Mewtwo with Pikachu lending him a helping hand.
The camera pans over back to Master Hand from a front facing view as it raises over him to reveal a glowing crimson orb. The camera switches over the to the orb which reveals a sinister face appearing from within it, something akin to Giygas from Earthbound. Master Hand rears his head/face/front facing area(?????) and sees the crimson orb slowly bulging as if something was trying to escape from it. Master Hand releases its own ball of yellow light that splits into several more and spreads it across each of the portals.
Meanwhile over at the Mushroom Kingdom where Mario is combatting Bowser Jr., Mario shoots a fireball to which Bowser Jr. hops out of his clown car with his hammer in hand attempting to swing at Mario. Before as Bowser Jr. gets his swing off, Mario readies a punch by swinging his arm backwards. Just the two land their hits, colliding their attacks, Bowser steps onto the field and lets out a mighty roar scaring both of them.
Bowser slowly walks, shaking the entire airship as he walks. Mario readies his fists as his old arch-nemesis finally enters the fray. Instead of attacking Mario however, Bowser points up to the sky. Both Mario and Bowser Jr. look up to the sky to see a large wormhole appear. From the wormhole, the small ball of yellow light emerges. Upon reaching the airship, it immediately transforms into a small little creature that looks very similar to a Primid but without the Subspace symbol, its face is a light yellow instead of purple, has tiny little angelic wings and a red top hat.
The creature which will be dubbed as an "Infitemite" (because it just sounded funny lol) hands Mario a letter while performing flamboyant flips and strikes a pose. Mario opens the letter first but Bowser Jr. gets greedy and snatches it from him showing it to his dad. Mario dizzy, looks up and sees the two have a shocked expression on their faces. Mario takes back the letter and eyes it over and reacts in the same way as his eyes gloss over the letter.
Across several other dimensions, the respective characters meet their own Infitemites with differently coloured top hats. What could be written in the letter that is invigorating all the characters of Super Smash Bros.? Mario drops the letter and runs jumping right into the portal. The suction is enough to pull him. Frustrated, Bowser runs after him but his jump isn't as high enough and falls off the airship. Bowser Jr. is quick to think and flies down as the camera shows him disappear into the space below him, then pulling Bowser up, albeit with difficulty, into the portal.
The camera soon pans over the letter. It reads as follows:
"Dear fighter,
You have been courteously invited to partake in the Wishing Stars Championship. I wish to see the strongest fight against each other in the world I have created. The champion who stands above the rest will be granted one wish of their choosing. May the best champion seek victory and prove the strength of their will before all else."
As Mario jumps through the wormhole, he imagines a gigantic festival at Peach's Castle while he proposes to Peach in his wedding attire. Bowser jumping in imagines a strong and powerful castle with a large armada by his side, Kirby envisions a peaceful Popstar free of strife, Donkey Kong pictures a ripe forest filled with bananas to his hearts content, Link sees himself with a powerful sword imbued with enough light to stop all malice and Pikachu envisioning companionship among others of his kind.
Several characters such as Wario, Samus, Yoshi, Cloud, Snake, Terry and others are seen jumping into their own portals as well in a rapid sequence until the screen glares white. Sparkles appear to form the title of the Adventure Mode, Wishing Stars. The two options New Game, Continue and Settings as the white background fades out to show a view of a large green and blue planet from outer space.
Tl;dr: Master Hand sends a bunch of reformed Primids to hand out invites for a large competition. The winner will receive one wish of their choosing and of course, who wouldn't want that? In the meantime, something sinister appears to be happening in the backdrop with a suspicious entity behind imprisonment in a crimson cage.