A few days ago I was actually pondering Galacta Knight as a boss.
Maybe a sort of Melee-esque adventure mode that usually ends with Master Hand & Crazy Hand rather than giant Bowser.
Meeting certain criteria, upon reaching Final Destination, Master Hand draws a little hole in the sky which he then peels off - kinda like how he tears a hole into the Final Battle in WoL. He'd chuckle a bit to himself as the crystal emerges, then Crazy Hand gives the crystal a hard flick of his finger, shattering it.
Galacta Knight gives a death glare to Crazy Hand, and Crazy then makes a beeline out of there. Master Hand just kinda stares, before getting sliced in half, a bit of swarm spilling out from him as he falls down below Final Destination. Cue the boss fight~