Megabug

MEGABUG

Eventually, Mario, the Rabbids, and their friends work their way to Bowser's Castle, where they'd been chasing Bowser Jr. for several levels. Upon arrival, they find Junior distraught that something had taken over his father and was now wreaking havoc upon the castle.

This is the Megabug, a glitch created by the Rabbids when they crossed over into the Mario universe, a glitch that started as a massive all-consuming vortex, but steadily took on a more physical destructive form thanks to all the warping being done by Bowser Jr. messing around with Spawny, until it found Bowser to be a suitable host for itself. A piece of it would survive and take possession of Rosalina in the sequel came to become Cursa.

As a fighter, Megabug will be large and powerful with lots of good aerial moves. Its moveset will pull from its boss battle while it is in possession of Bowser, as well as other notable features it displays outside of battle.

Likelihood rating: Never gonna happen.