Doc Benton (Supernatural) regularly transplants others' organs and body parts onto himself.
In addition to replacing his left hand with a mechanical one and installing a clockwork heart, Karl Ruprecht Kroenen (Hellboy/BPRD), suffering from severe "surgical addiction," performed surgery on himself to remove his own eyelids, lips, and finger and toe nails.
Hiei (Yu Yu Hakusho) had his Jagan Eye implanted in his forehead by a demonic surgeon.
As a result of an experimental medical procedure, Peyton Westlake (Darkman) no longer has any nerve endings, rendering him immune to physical pain, while also causing adrenaline to flow through his body unchecked, increasing his physical attributes and leaving him emotionally unstable.
Upon his initial demise, Colonel Volgin's (Metal Gear) body was taken into Soviet custody and modified extensively, turning him into the nigh-unstoppable Man on Fire.
Professor Pyg's Dollotrons (DC Comics) are once-ordinary people who were modified by extensive surgery.
As a result of an organ transplant from a ghoul, Ken Kaneki (Tokyo Ghoul) was transformed into a human/ghoul hybrid.
Saya (Saya no Uta)
Touhou Fuhai (Rosario + Vampire) is the inventor of the Human Modification Technique, which reconstructs a human's cells to enable them to use yōjutsu...
...of which Tsukune Aono is the first success.
Chimera (Marchen Awakens Romance) used the power of Ghost ÄRM to replace parts of her body shredded apart during torture.
Dr. Genus (Onepunch Man) is such an accomplished intellect in bio-molecular enhancement...
that he was able to restore his lost youth among a host of other feats accomplished.