It sounds weirder on paper but...like Mario is non-violently split in half and his bottom replaced by Goomba legs as Bowser steals Mario's bottom half because that's his strongest power. Also Bowser is pirate themed here so there's a "booty" pun.
Donkey Kong finds him and since Mario is unable to do his usual athletics, he hitches a ride on DK (think Baby Mario riding on Yoshi or Diddy Kong riding DK in DKCR). There's layers here!
And Donkey Kong controls like 3D World style, but you have a large open hub world (an ocean with many islands, DK Island included) that lead into obstacle course levels with a variety of missions...think a larger scale Bowser's Fury.
And some various enemies have also been split in half by Bowser, so their bottom half wander around and Mario can attach his top half to them...like a spring-tailed enemy to bounce around, a wiggler body to climb up walls, etc. Also in his stubby Goomba-leg form DK can toss him around and he can go through small passages and other such stuff, as well as being the one to utilize powerups like Fire Flowers and Boomerangs while riding DK.
Just came up with this idea after playing Bowser's Fury so it's still cooking but I got a good idea going here.
Even got the perfect subtitle...Top Bananas. Y'know because DK loves bananas, Mario is just his top half in this, and they're Nintendo's two big stars.
