Did you not play Mother 3? The Masked Man's techniques, if I recall correctly, consist of sword swipes, arm cannon shots, PK Thunder, and PK Love. He also has wings that he uses to fly away a few times. None of these wow me as much as they seem to wow others.
"But you can have them all together! Think of the tools!"
Somebody is getting ready to say that, as they read this, and, well, I can't really argue with that kind of logic, but the same could be said for all proposed characters.
Hm. I promised myself I wouldn't criticize your downvote, because I can get a little defensive of my choices, but here's where I don't follow your logic:
Besides, Masked Man only has, what, two attacks? PK Thunder and sword attacks, I believe, except for the final battle where he also can use PK Love.
This is incorrect. PK Love is used in both battles with him, and he obviously knows it throughout the game because he can pull needles, so I don't get how why you're downplaying it by saying it's a final battle only thing. As stated, Claus obviously possesses some form of swordsmanship, so not all sword attacks will be identical. Claus has an affinity for lightning, but I don't think it's ever described as PK thunder. Not to mention, he also uses the Shield Killer in battle and a bomb during a cutscene.
In contrast, Lucas has: PK Love, support PSI, hit with stick in-game.
The problem is you seem to be painting the moveset in the absolute worst light possible, whereas the same issue is ignored for a few of your upvotes. Just calling swordplay "sword swipes" completely ignores that his sword adds plenty of different moves, such as a stab, a slash, or the jump attack he uses before the first battle. Plenty of characters from RPGs were given this sort of treatment, or borrow moves as you've suggested for Giegue, so acting as if it exclusively makes Claus bland is flawed.
An arm cannon adds plenty of moves in the vein of Samus's aerials and the sort of playstyle you've envisioned for Pico, as well.
So, Masked Man has physical attacks, like Teddy, technology like Jeff, and PSI like Giygas/Giegue.
I can see how it might not wow you, but calling Claus's moveset potential shallow is a bit of a stretch.
Masked Man - The main reason for me opposing this character is stated in the OP: He is a walking spoiler. I can speak from experience, Project M convinced me to try out something I've never played before, namely Castlevania. It's been said that Lucas can wear the (Un)Masked Man's outfit in P:M 3.0. That is fine
Consider it like this, anyone who bothers to play Mother 3 after they play P:M will have seen Lucas wearing the Masked Man's clothes and using his sword, and possibly read Lucas's trophy, which states that he crosses swords with his missing brother, Claus. Why is such a dead giveaway okay but the character (who doesn't even have to be directly revealed as Claus) unacceptable?
However, I do not feel that Mother needs another representative, and especially not two from the same game
I wouldn't be adverse to Giegue, or Loid, or Jeff, or Teddy.
Why do you cite over-representation as an issue if it obviously isn't really an issue for you?