I mean, even having Master Hand in just the Multi-Man Mode is good enough. He's not designed to be a typical fighter in any way. He's still a boss and has unique boss rules.
That would be a great option and they could add more of his moveset than what was in World of Light. It'd be a "mode update" instead of a proper new fighter, and not a new mode either, so Sakurai could legit get away with it through "exact words". Though to be fair, he meant there's 6 character packs, and MH would not be part of a pack either way. Since if he meant straight characters, Mythra would already change the meaning to be more confusing than his intention.
Sadly I don't think it's happening, but it would be nice. I've played him I think three times now overall in the games. Smash 64 being a Gameshark, Melee via the Action Replay, and properly in World of Light. I never did the Melee glitch to use him, though. He's fun, but I like him best in 64 and Ultimate where you can move him around and his input commands are the same kind as regular characters(Up A, etc.)