Most superheros spend their lives fighting evil transhumanist villains. They don't tend to be transhumanist themselves, and their superpowers are (in Western tradition) gained by a nonreproducible accident.
Except the archetypal superhero, "superman", whose powers are reproducible in the literal sense, including to a significant degree with humans. (It is notable that Clark himself doesn't go out of his way to reproduce half-superbabies. In fact that sounds like something a villain might do and so is quite likely the plot of a comic somewhere.)
The next most famous superhero "Batman" also has powers that are reproducible. He has ridiculous amounts of money, a ridiculous amount of training and practice, an engineering department and brilliant foresight and planning. All of these are 'just' hard work and so are reproducible. In fact "The Batman" role even changes hands at times, which means it could be reproduced.
The instantiation of The Hulk I'm most familiar with quite possibly could be considered a transhumanist. Sure, he got his Hulk powers through an accident but that was an accident while actively trying to make himself transhuman through his genetic engineering.
Iron man. That's not just reproducible. That's mass-reproducible. In fact the most irritating thing about "The Avengers" is that Stark isn't going to take the obvious next step: Make a goddam suit for Scarlett Johansson! Her agility and combat prowess would be perfect for controlling a suitably remodeled Iron Man suit---where that suit would offset her vulnerability and relative weakness.
Then there is "Captain America". Wasn't the "accident" there that the super-soldier enhancement program was destroyed, leaving only one? He was actively created as part of a (military run) 'transhumanist' research program.
Then, again on the 'anti-reproducible status quo preservation' side of things there is "X-Men" where one of the movies has trying to reproduce superpowers as the plot by the Villain that needs to be thwarted.
In fact the most irritating thing about "The Avengers" is that Stark isn't going to take the obvious next step: Make a goddam suit for Scarlett Johansson!
This actually happened in the Ultimates comic, an alternate version of the Avengers.
I just watched this, a very pretty version of "don't try to make yourself different, just accept who you are", and I realized that self-directed change in fiction is a worthwhile topic.
What I'm looking for is stories where main characters change themselves in ways which are basically improvements-- getting beyond the usual human is a plus, but for purposes of this discussion I'm including any significant positive change.
Another big plus would be the character needing to learn which of their goals make sense, and which methods work.
**ETA:** That was a bit of a stub-- HPMOR is partly about Harry and Hermione changing themselves, generally for the better I think (I've only read it once). It would be interesting to see what happens if Quirrell decides he needs to upgrade himself.
*Stranger in a Strange Land* is an interesting partial example-- the Martian language is presumably an upgrade for the human race, but it was developed by and for Martians, and needs some modification.
A *lot* of relatively recent fiction has people learning martial arts. I think appearance makeovers (typically for women) have become less common. I don't think there's a lot of fiction about appearance makeovers for men-- *The Stars My Destination* has one, but it's offstage. It wouldn't surprise me if *The Count of Monte Cristo* (frequently referenced with TSMD) has one.