Yeah, Batman has been explored in many ways that seem to stretch aspects of the character to the fullest, but the logical story arcs of a Superman have rarely been touched on.
As far as fully realized, I think there is a ton of room for more characterisation for both characters. Maybe someone like Busiek, who among the 2 current interesting Superman writers, has the best gift for character and voice, will do something with that.
And as for Swamp Thing and what constitutes a superhero, I found this new definition by Peter Coogan, an academic who has written a book called the Secret Origin of the Superhero:
Superhero (soo'per hîr'o) n., pl. -roes. 1. A heroic character with a selfless, pro-social mission; who possesses superpowers-extraordinary
abilities, advanced technology, or highly developed physical and/or mental skills (including mystical abilities); who has a superhero identity embodied in a codename and iconic costume, which typically express his biography or character, powers, and origin (transformation from ordinary person to superhero); and is generically distinct, i.e. can be distinguished from characters of related genres (fantasy, science fiction, detective, etc.) by a preponderance of generic conventions. Often superheroes have dual
identities, the ordinary one of which is usually a closely guarded secret.
-superheroic, adj. Also super hero, super-hero.