One 'transhuman' variant is the people who download into computers and live on as virtual intelligences or whatever, perhaps even hopping from body to body.
Magic Jar is pretty much this. A magic jar item, with a repository of cloned bodies (or even simulacra of various species, including dragons) that the magic-jar-user could 'download' into could make for an interesting 'Today I Choose Faces' scenario, where the spirit-people's true bodies have long ago died, and they hop from body to body, depending on their whims or needs of the moment, having perhaps grown 'faded' from the process and forgotten their original races, genders, etc. Massive 'soul-repositories' could function like oracles, consulted by others, as the vast metal armillary spheres whirl and click, adorned with thousands of magic jars, each containing a soul that has transcended the flesh and is now part of the great mechanism, in constant communion with the others, in a vast sea of shared memory, experience and artificially crafted sensation (through user-tweakable programmed illusion spells that are only visible to those outside the machine as images flickering across the surfaces of the magic jar gems themselves), part of an increasingly hive-mind-like 'community.'
Transhumans who have themselves modified to different climates, including the deep sea, or outer space, or whatever, would be replaced by sahuagin, who *are* the modified deep sea humans. Anyone can pay to have themselves polymorphed temporarily to visit the Deep Cities, but the natives, who were born in this form, or have permanantly transformed themselves, regard such visitors as 'tourists' and look upon them with benign contempt (while pragmatically taking their tourist coin). The Deep Ones have their own culture, pretty much made up out of whole cloth, to differentiate themselves from the surface races they used to be composed of, and recognize aquatic elves, koapoacinth, etc. as one flesh, united in their aquatic experience, while shunning ties to the surface races they once were a part of.
Similar transhuman adaptations could exist in other planes. The Azer could be refluffed as a group of dwarves who used powerful magic (and perhaps some never-to-be-spoken-of dalliances with genie-folk) to adapt themselves to be able to function in the blistering deadly heart of Elemental Fire itself. They refuse to admit whatever boon they owe the Efreeti for this transformation, but it's pretty much common knowledge (and equally common knowledge that to bring it up in front of a Transdwarven of the 'Azer Clan' is to invite a metallic fist to one's breadbasket...).
Some races might avoid 'modding' themselves, and, with the exception of the Azer, dwarves might consider the whole notion suspect, despite being all-too-willing to magically warforge the heck out of themselves with 'Golem Grafts,' while elves have adapted to undersea life (aquatic elves), underground life (dark elves) and even aerial life (winged elves!), being consummate crafters of their own flesh.
If the setting has a necromancy-friendly kingdom, transcending mortal limitations might come in all different flavors, with ghouldom and, amusingly, vampire status being the literal 'cheap seats,' as the transformed retains mortal hungers and some hefty vulnerabilities, as well as a hefty debt of service to be paid to the ghoul or vampire who transformed them. The true elite try to arrange for their transformation into mummies or liches (if they have the ability), but life as a mortal working their way up to that status is difficult in a land ruled by those who have already 'crossed over,' including ghouls and vampires who don't really want a whole bunch more mummies and liches topping them on the social status charts, and the already existing mummies and liches who may not be keen on seeing too much new competition, since there is never any 'room at the top,' when the 'boss' never dies... Such a society would find itself *needing* to expand, at the detriment of it's neighbors, both to provide fodder for those undead who still need to feed (ghouls gotta eat, after all!), and new territories to be claimed by the ever-increasing numbers of the undying upper class.
The whole soul-hopping Magic Jar + Simulacrum thing mentioned above might have been perfected in this land, and be one of the few 'acceptable' marketed products they can export to other lands. "You too can fly on a dragon's wings! If your credit is approved, you can even try a test flight, but remember, you break it, you bought it!"
Technically, the Outer Planes pretty much already *are* Transhumanism at it's finest. Human spirits move past the flesh and blood existence of the Material Plane and become petitioners (very similar to virtual intelligences), and then are upgraded over time into archons, devils, etc. Very Post-Human. Taking it a step further, *all* of the sentient otherplaner races could be transcended humans (or mortals, anyway), including Elementals, Genies, etc. Ancient societies could have given themselves over to the four elements, and their 'petitioners' live on as elemental souls, pure elemental energy, growing over the aeons into the greater and elder elementals, becoming more coldly alien, and forgetting their mortal existences in the world of flesh and blood, pain and hunger, entirely.
