I forgot that I never threw out a trailer idea.
Here's my humble attempt:
We open on a jungle, in an open field area, Bowser is terrorizing Mario, Pikachu, and Yoshi. Bowser seems to have the upper hand, but DK and Diddy come swinging out of the forest ready to help, but something else is coming right behind them. It lands beside them....
it's Dixie Kong! Cut to some gameplay footage showing she's a semi-clone of Diddy using some of his moves like sideB and downB but also has her own hair moves and such. After this, we cut back to Bowser, who's looking more worn out and beat up, with the heroes having the upper hand. From a side view of Bowser, suddenly, a finger taps him on the shoulder. He looks back, a blunderbuss rears into view and shoots a cannonball, knocking Pikachu away. Cut back to a smoking blunderbuss, zoom out to see....
"King K. Rool Krushes the Kompetition!" He lunges at the heroes and it cuts to the gnarliest, most ridiculous gameplay footage you've ever seen. K. Rool is 4x the size of Ridley, his jab alone does 70% damage, his neutralB plays fake credits where every character onscreen has to stop what they're doing and watch as it takes 45 seconds (this move has absolutely cooldown and can be spammed as many times as you want), his downB brings Grant Kirkhope onto the stage to compose more music for us for 20 minutes, he cannot double jump or it will immediately KO him due to being so big, and his Final Smash just cuts to the victory screen with K. Rool as the winner, regardless of circumstances. Cut back to everyone defeated, K. Rool looks at the screen and says in his voice from that Donkey Kong Country cartoon, "BRING ME THE CRYSTAL COCONUT, YOU LUNKHEADS!"
Suicide rates worldwide drop to 0%, Sakurai is awarded a Nobel Peace Prize, all Smash players set aside their differences and play K. Rool only Round Robin tournaments for the eternity, and Waluigi is still an assist trophy.
Obviously it's all wishful thinking, but I think this would be the most realistic outcome if K. Rool is included.