Well, first off, Masked Man has a pretty wide variety of tools in his arsenal that would set him apart from Lucas and Ness, and change the staple(to Smash bros. fans) of Mother only being about Psychic children(and yes, i know MM has Psychic powers himself, but let me go over his other moves).
He has his Laser Cannon and Lightsword, along with the Shield Killer. He could use the Cannon and shield killer as special moves, while the lightsword could be incorporated into his smash, normal ground/aerial attack and tilts. He also has his wings which could let him have multiple jumps and good aerial game. He also has Psychic powers, and he could probably incorporate them into some of his attacks like Lucas heavily does, and his Final Smash could be his One Hit KO Lightning move.
Sure, we could have Kumatora, but i mean, Lucas basically has most of her offensive PSI moves, and her having more offensive PSI than Lucas was the main gimmick about her gameplay wise in Mother 3. There's not really much Sakurai could do with her that hasn't already been done with Lucas and Ness. Duster on the other hand, could probably be a more unique character with all of his thief tools, although i honestly would prefer Jeff or Masked Man.
As for the "Walking spoiler thing", as long as Sakurai doesn't say anything about his relation to Lucas in his trophy description, or his true identity(he could hint at it, but if he doesn't directly say it, it'd be fine), then there's not much of a problem. I mean, the only thing that would give away his connection to Lucas other than that would be if he took his mask off as a taunt or something. Also, another reason alot of people want MM over some other Mother representatives is because people would rather have a villain Mother rep. rather than just another Protagonist(which is what the majority of the candidates for Mother reps. are). Sure, there's Porky, but he was already a boss in Brawl, i kinda doubt Sakurai would use him, even if he is a valid candidate.