I made a poll about new Kirby Bosses in Smash Ultimate that could replace Marx, and I want to elaborate on how their fights would work.
Galacta Knight, for the first phase, you'd have a fight with Meta Knight using the Galacta Knight costume and Spirit, then the real deal would begin, and you'd fight a truncated version of the Super Star Ultra battle, the first phase can easily be cut, but I just wanted to touch on it.
Dark Matter, I'd probably go with an on-foot recreation of Hyper Zone 1 and 2, that's about it.
Masked Dedede, this would be a Giga Bowser type fight, using similar attacks, but more dangerous, along with some exclusive attacks from SSU.
DynaBlade, really similar to the original fight, once again, this would make for a better assist trophy.
Nightmare, a more elaborate take on the Parallel Nightmare fights, complete with the revenge form, or a Power Orb phase instead.