That was Mephistopheles, who can't be copyrighted because it's a mythical character. Marvel's version, Mephisto, was on a list of forbidden characters that Ike Perlmutter didn't want any artists or writers using because he was part of the FF rights. If you check any of the credits, you'll see he's clearly listed as Mephistopheles.
I read many variations of this idea during production of Fant4stic, but I didn't buy it then and I don't buy it now for several reasons.
First, none of us can explain
exactly why Dr. Doom became an iconic character who has endured for 55 years. Ask ten people and they'll give you ten different reasons, because he's a multi-faceted character that appeals to different people for different reasons. Even Stan Lee or Jack Kirby can't tell you exactly why he became as big as he did. They created thousands of villains, but, for whatever reasons, his combination of characteristics really clicked while the others didn't. The best writers in the world can't simply create a new character who will work in the same way.
Secondly, a character like Dr. Doom crosses lines between geeks and 'regular people'. He's such a well-known character that most people have seen him in some form. They've seen him on T-Shirts, posters, cartoons etc. They've even seen (poorly rendered) versions of him in Fantastic Four films. That kind of broad recognition is why characters like Spider-Man, Superman, Batman etc. will always be bigger than some of the minor characters.
And lastly, us geeks do make a difference. We are the ones who create buzz. We talk about these things long before the film comes out, and that spills over to the people around us (even those who don't know and don't care). Those people may not know much about a film, but they know they've heard people like us saying either positive or negative things. And then we're the ones who show up on opening night (for the films that excite us). Regular people see us standing in lines and wonder what all the fuss is about. We drive opening weekend box-office numbers and those numbers spread the publicity. We make the films number one and then that number one ranking is seen by others. And we go back. We continue to keep the box office up and the publicity rolling. We're the Typhoid Mary that helps spread the disease.
The factors above are why some of these properties are so lucrative. If they weren't factors, anybody could make a blockbuster film. Just create a new character and do what Marvel does and you've got a hit film.