Well they managed to make Professor X work in one X-Men fighting game by literally having him do wheelies in the chair, so not impossible.
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Honestly I think I'd go the Ganondorf route and just have them show off their sword in a taunt. While I think the "too many anime swordsmen!" complaint is a bit overblown, there really isn't much unique SMT protagonists could do with a sword. What make SMT SMT is demon summoning (well unless your Digital Devil Saga), I think I would be fine with the demons being incorporated into every move and having the characters be something more like a summoner archtype. You have a interesting mechanic right there to fully fill out a moveset, a sword (and even the gun/magic) would just be a bit blasé.
Well that's kind of the entire flaw in the "no non video game characters" argument. The truth about Smash is that nothing is off the table, characters will get a pass if they deserve one. No, Sakurai isn't about to put Spongbob or Hank Hill in all willy nilly, but if there's a great character that happened to originate outside a video game then they will get a pass. Hell Dracula is already in the game as a boss! And as a literary character he's barely older than Mickey Mouse. Neigh sayers will counter with "That's not Bram Stoker's Dracula, that's Castlevannia's Dracula" when they are literally the same character acting the same way doing the same things. The same "that's not [non video game character] that's [video game version of non video game character]" logic could apply to pretty much any character someone could reasonably want because everyone has been in a video game. If Mike Tyson's and Nintendo kept in good terms and Nintendo made a bunch more successful Punch Out games with Mike Tyson in them then Mike Tyson would legitimately be a decent character to put in the game as a second Punch Out character. Sakurai's rule of "no non video game characters" isn't some kind of holy law that needs to be obeyed. It's a guiding principle. The only rule is "if Sakurai (or whomever else is in charge) thinks it's a good idea". If a character that originated outside of a video game presents itself then they could get in. The guy from the Witcher, Jearlt or whatever his name, might actually get in some day. A technicality of originating in a book series won't be an impediment (though like all third party characters, licensing might). Hell Lucario is technically a movie character.