(This post could be muuch longer. Plus, it may be a bunch of baloney. Sometimes I like to write things out to see how I feel about them...)
I believe that if a person isn't a fan of a particular subject, they only have a set amount of room in their brain for it. For example, I'm not really interested in vehicles types. To me there are cars, trucks, and vans (and maybe SUV's). To the general public, superheroes are the same way. They only have room for so many categories in their heads.
The three most famous superheroes cover the three distinctive looks mask-wise. Often superheroes "icons" are discussed as what type of character they are, but people are so visually based, I think the superhero's looks has a great deal to do with their fame. If Spider-Man looked like Peter Parker with a "Robin" mask, would he had become as famous? I personally don't think so.
You might argue that other memorable heroes also have partial masks. Captain America does. Wolverine does (or, sometimes, no mask). Powers and personalities aside, I would maintain that it's not their masks that allow their fame. They would both merely be "Batman" if they didn't have the notable accessories of a shield and claws, respectfully.
When I think of other rather famous heroes, they also fall into unique visual "categories". Hulk covers the "monster" category. Wonder Woman covers "females". (Sorry, but superheroes seem to be a boy's club. If you don't accept that, you could put her in a "tiara" category, I suppose.)
Can Iron Man obtain major, long-lasting fame? Maybe. We really don't have a hero in the "robot" category yet. (Well. if he isn't overshadowed by Transformers.)
I believe it would be difficult to create a new, enduring, famous hero without creating a new "category". Does have Thor (and his hammer) have a chance? With the right presentation, the Human Torch might have a shot. (Or is he merely "monster"?) For a new character to hit it really big, he would have to have an entirely new look. Something like a headless midget. I don't think that category is taken.