Well, many people agreed that Batman has a number of extropian/transhuman qualities, but what about Iron Man? High tech armor for every occasion, implants, he was even an upload for a while.
Regretably, most trans/posthumans one encounters in comics are likely to be villains; Galactus is the best example of this. Also, any character who actively tries to become more than human is usually the bad (or at least misguided) guy; perhaps this is why the heroes origins are accidents. The operative meme seems to be "no one who seeks such power can be trusted with it".
Anyway, there is one comic book out that deals with the possibilities of
ideas. It's called Transmetropolitan, written by Warren Ellis, as part of the DC Vertigo line. The
stories don't always paint a bright picture of this future, but it certainly is a dynamic one, viewed through the eye of Spider Jerusalem, a Hunter S Thompson on smart drugs, in his weekly column "I Hate It Here". It's also funny as hell, if rife with profanity & political rants. I recommend it