Ultimo.
He was used in the Iron man 2 videogame but the movie tie-in games have dubious ties to the MCU and should be taken with a grain of salt. Ultimo was also used in an MCU tie-in comic as he was made out of the left over Ultron from Sokovia, but that's still pretty much Ultimo in name only. He's still yet to make a proper appearance in the MCU as far as the films go.
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For anyone that needs to know why OMD is awful please search the internet for Linkara' s video's specifically his One more day review or his One more day Analysis.
Well her big thing with Tony was that she was the Catwoman and Talia to his Bruce. And that worked for a long time because Tony was damaged enough and unable to keep a relationship with regular girls that Masque had an attraction for Tony. This is also the big reason why Bruce/Selina is a thing since both are characters who gave up on "normal" a long time ago and can't accommodate sane and reasonable people in their lives. But in the MCU, Tony and Pepper are soulmates basically. So you can't make that Masque thing work. And then they kinda did that in parts with Maya Hansen in Iron Man 3, so a big part of who she was is gone. Also RDJ's Tony is pushing 50 at this point and having Masque being a young villainess love-interest against him wouldn't fly in the Disney era. James Bond can do that because he's all European and dates Europeans but not Iron Man, mentor of Spider-Man.
Masque has a cool dynamic with Kate Bishop and she felt really scary in the Fraction stories. Incidentally, Fraction used Masque in his Man Hunt storyline where Norman Osborn during Dark Reign takes over SHIELD, and in that Masque goes against Pepper's Rescue, and he had Pepper beat up Masque and hijack her costume to dress as her, and he later repeated that same beat in his Hawkeye series with Katie Kate doing that to her. So even Fraction wasn't really using Masque as a villain against Tony specifically.
There are a few that pop into mind but they might be my essential Marvel villains as opposed to THE essential Marvel villains.
Amora
Gorr, God Butcher
Mangog
Modok
Morgan Le Fay
Nightmare
Mephisto
The Hood
Moonstone
Void
Maybe. I mean obviously different strokes and whatnot. But Fraction's Hawkeye run, which I am sure will influence somewhat the upcoming Disney Plus series, did use her in a new way and story, and for a lot of people, Masque is a Kate Bishop villain especially since she goes to great lengths to try and kill her.
She could also work as a Black Panther villain, since T'Challa is supposed to be in the MCU a kind of James Bond type character, and so on.
Ignore this post. Accidental repeat.
The MCU doesn't have Marvel Comics' current top villain, the Foreigner!
I don't blind date I make the direct market vibrate
With MASTER OF KUNG-FU in the works. And just watching THE WINTER SOLDIER tonight. I couldn't help but think of the unexpected bonus of longtime Cap nemesis. Batroc The Leaper's appearance early on in the flick. He'd be perfect to show up in Shang-Chi's cinematic debut. Along with members of his Brigade of merry mercs. Machete. Zaran The Weapons Master, who started out as a Shang-Chi foe. Even the Swordsman.