I actually prefer Smash without any 3rd party characters whatsoever. Now that the floodgates have opened though, I just think that the Minecraft would be a way bigger deal than Cloud, Snake, Simon, MegaMan, etc, (not Ryu, Sonic, Pacman though), and he would make sense considering how much Minecraft support Nintendo consoles have gotten.
Still though, I'd like the majority to still be Nintendo characters.
Octolings, Pearl/Marina, Rex/Pyra, Isaac, Wonder Red, Spring Man, Elma, Incineroar, Tom Nook, Hades, would all be perfect! I don't mind Kirby getting another character (after all, I do have Kid Icarus getting another one, and that's way more niche), but I'd prefer anyone else over the Bandana guy.
Nintendo alone has so many characters they can still add, and each of them would feel totally valid.