More iterations of Nintendo's mascot Smash Bros. does not need, but hear us out.
Super Mario Odyssey's core gameplay mechanic is good. So good, that Nintendo should take every opportunity it has to reuse the same or similar concept in other games. The same old Mario he may be, but with Kappy protecting his hair-do as opposed to a bog-standard red cap, this proposed offshoot would discard his usual stock of abilities in favour of 'borrowing' different forms.
Bullet Bill, Chain Chompa, Koopa and Hammer Bro transformations, to name just a few, could take the place of Mario's usual moveset and his Final Smash, of course, would seem him blow up in size to become the moustachioed T-Rex from Odyssey's Cascade Kingdom
There's always the option of having Kappy be a standalone character too, but Nintendo's been obtuse in the past when explaining how the ability works, so we'll let it work out the finer details for itself.