There have always been superheroes in fantasy, going all the way back to Greek myth. Hercules, Odysseus, and Perseus all fit the general definition. They just aren't called such, but a rose by any other name...
I actually loathe superheroes in general, but this is an interesting question that has made me think:As LOTLOF has said, there have always been superheroes in fantasy. A big question is: How much of comics superheroes do you want to use in your story.
No, because I think it's ridiculous and is the main reason why I loathe superheroes.
- In most comics the superheroes wear outlandish, skin-tight, and colorful costumes that practically shout Look at me. Do you want that? And why?
This is a comics / cartoons / actions figures convention and is stupid in literature. *Except*, when you think about it, it's actually quite prevelant in literature. It's not as blantant, obvously, but I can think of an awful lot of instances where it's there if you really think about the character descriptions.
- In most comics the male superheroes have body-builder physiques, the females have Playboy bunny physiques. Do you want that? And why?
No, *Except*, when you think about it, there often are. Maybe not in more recent fantasy as much, but what's Sauron if not the ultimate supervillain?
- In most comics there are supervillains for the superheroes to fight. Do you want that? And why?
No, this is also stupid. Except Gandalf had one, sort of, and so does Bayaz. Hmmmm....
- Do you want your superhero to have a secret identity?
No, we've already got a lot more than I thought we had!
- Are there other parts of the comics superhero experience you want to include in your fantasy?
Comic heroes have good bodies because they're fit. Have to be fit to fight crime, and fighting crime burns a lot of calories (both in exertion and because a lot of calories go into repairing the body after you got your ass beat).
Steph Swainston's 'Fourlands' series, starting with The Year of Our War, features a bunch of basically superpowered immortals in a fantasy setting iirc.