I've mentioned it before, but on the topic of bosses I like the idea of a Melee-esque adventure mode. I'll go off of the context of Perkilator's structure since I like it a lot, but with this mode being there.
The final battle of this Adventure Mode is Master Hand & Crazy Hand, the latter only appearing on higher difficulties. Master Core, appearing elsewhere, does not emerge under any circumstances. Instead, on Very Hard, and if you reached the hands within a certain amount of time (such that it might take a bit of practice to make the time consistently, but nothing too crazy) then the fight against the hands ends prematurely halfway through, Master Hand drawing a hole into the background with his finger and peeling it off to reveal a wormhole.
Master Hand suddenly gets torn wide open by who emerges, and that same individual blasts Crazy Hand before he can react. FD's background warps into a reddish-pink dimensional spiral and the true final fight begins with the strongest warrior in the galaxy.
Galacta Knight appears significantly larger than Meta Knight, in reference to how he appears as Aeon Hero in Super Kirby Clash - around Ganondorf's height while being about as wide, seeing as he's a sphere. He's extremely mobile and hits extremely hard and even has combo strings, which Smash bosses almost never do. He has perpetual super armor, too, but strong attacks will break said armor for long enough to get some good hits in yourself, provided you properly parry his continued rushdown. He can jump as many times as he wants and always is treated as though he's at 0%, to prevent any cheese (like other bosses, he has a health bar.) On the other hand, he's an extreme fast-faller and so if you can manage to get him in hitstun - no easy feat - you'll get some good reward.
Obviously, he's not a character you'd be able to play as, but strictly a boss fighter.
Though maybe you could play as him via a button combination, but only after beating him as a boss, and ONLY in offline Smash mode and online in friend-only arenas. Never in any serious contexts. Common sense suggests he'd also be banned from IRL tournaments too.