As an aspiring writer and video game developer of my own some day, I've thought many times about Nintendo games/pre-existing series games I myself would make that will probably never happen.
I had an idea once of a brand new 3D Mario with the premise that Bowser and K. Rool teamed up to help each other achieve their own goals, and mixing their armies and technologies to make an unstoppable army.
Meanwhile, Mario has to team up with his long time rival and unlikely ally, Donkey Kong, to save Peach... and like a thousand bananas... (And maybe Diddy too if K. Rool is feeling particularly merciless that day). Cue humorous cutscenes of Mario and DK butting heads and arguing in pseudo Italian gibberish and "ooh ooh ooh" as DK is as stubborn and brash as ever, contrasting Mario's less aggressive, more item and object based adventures, but DK acts as the partner of this game, a la Luma and Cappy, chucking Mario around, using him as a blunt weapon to hit things with, cargo throwing Mario like a barrel into a rolling attack, etc. Maybe there's a partner switch out system like in the DKC games, where you can play as DK or Mario, each with their own advantages and disadvantages. Mario jumps, DK smashes walls, Fire Flower VS. Coconut Gun, etc.
Meanwhile, Bowser and K. Rool are butting heads just as often, with K. Rools cruel machinations and long term planning colliding with Bowsers head on approach, but they still work together in multiple boss fights, probably the standard formula of a Bowser fight repeated thrice throughout the game with increasing difficulty and probably a new phase during the final boss, but this time it's some combination of Bowser and K. Rool. Something crazy like them sharing Bowser's clown car, but it has a Krown now and has "Klown Kar" painted on the front in big letters, meanwhile Bowser is firing fireballs and Koopa shells and Bullet Bills from K. Rools blunderbuss as K. Rool throws Mecha Koopas or something.
Most if not all bosses could combine Bowser and K. Rool's armies as well. Imagine a fight where Kamek enchants a Krusha to make him really big and roided out. Or have a large Monty Mole throwing large wrenches, but you get a free hit to get his attention, from then which Kleever knocks the wrench out of the Mole's hand and lets the mole throw the haunted sword, from which the real boss battle begins, just crazy stuff like that.
I hope I don't make anyone sad by tantalizing them with a game that's probably not happening. Hey, maybe if all the stars and planets align on a blue moon during the eclipse, this idea will be like that story of that guy who posted on a forum saying his dream is to work at Nintendo, and then years later that guy's name appeared in the Breath of the Wild credits. Writers are always told to hold onto their ideas, just in case.
Now excuse me as I copy this into a word document now, and probably add to it as I'm getting my excited itch to write again.