Answer:
I’m sorry, but this whole line of reasoning is a fallacy.
Every character is going to have their fans. There’s going to be something in every possible character that appeals to a certain number of people. And of course those people are going to want to see more of them.
But popular characters aren’t popular because they make so many appearances. Popular characters make so many appearances because they are popular. More people like and respond to those characters than to other characters.
With any character, what you really need for them to show up is creators who want to use them and have stories to tell regarding them. (And also a venue that makes sense. If you’re writing DAREDEVIL, it may be difficult, for example, to find reasons to bring The Abomination into the storyline again and again.) But more popular characters are going to have that more often, because those characters are simultaneously more popular with creators as well as fans.
And the idea that there are great characters who aren’t being used who’d rocket in popularity to the top of the charts if we’d just feature them in some stories doesn’t seem to have a whole lot of basis in reality. A creative team with the right mojo can sometimes make a character more popular, sure—but I can’t offhand think of many examples of characters that were obscure who were later brought back and suddenly exploded everywhere in a meaningful way. Typically, there’s more going on in those cases than just that.
(Plus, try to get any ten fans to agree on which are the great characters. The guys you really love and want to see more of are of no interest to the guy next to you who’s reading this answer, who has his or her own list of characters that you barely know or care about.)