A confluence of things bring me to the idea that shows up in a number of comics. One of them is the podcast Geeks with Beer, specifically the episode where one of them (Frank?) describes the plot of Arkham Knight. Another is an adventure I ran in ICONS involving a certain villain ripping his heart out for his beloved. (He'd done it before. He got better. She was tired of the gesture.)
The idea is that regenerating tissue eventually takes over the body that it's put into. And that might even be true but mostly it's an old horror story trope (see Hands of Orlac or Mad Love--the 1936 movie--or even Eyes of Laura Mars or listen to the episode of Lights Out entitled "The Chicken Heart That Ate the World"--I think; it might have only eaten New York).
All of them hold the idea that the badness is inherent in the tissue itself, not in the recipient. In comics, the organ literally takes over the person, whether it's the Joker's tainted blood or some piece of Bruce Banner that's continually growing inside you (which makes the whole She-Hulk thing particularly nasty to think about: perhaps Jennifer Walters has very little in the way of powers; it's just Hulkness growing in her. Eventually she won't be some zaftig or Junoesque superheroine...she'll just look like Hulk, because he will have taken over).
Really, a lot of stuff in comics works through some kind of sympathetic magic or contagious magic, even though it often has a pseudo-scientific explanation. Superman is perhaps the most constantly scientific of the early heroes and his powers lapse into obsolete theories once in a while (the whole heat vision and X-ray thing seems based on the obsolete theory of sight that we emitted something that we then detected).
Much of the time there's a kind of morality play: when someone gets the heart of a killer in order to tame the urge and put the killer to good use, it's rare that the person succeeds: instead, it's a message that there are Things We Are Not Meant to Know. Heroes are rare and people with powers generally succumb to their darker urges (possibly because you only need one hero, but you need twelve months of villains to fight).
(Side note: I wonder if you could do a small adventure or campaign involving the characters who have decided to actually change the world, and who are inevitably thought of as villains because of that. So the opponents are either lawyers and bureaucrats of the campaign city, or all the heroes who are trying to maintain the status quo. Heck, you could have more than one adventure where the PCs get away because the "heroes" end up fighting each other for the chance to beat them up. I have often been uncomfortable with the knowledge that Batman, one of my favourite heroes, is essentially part of the 1%, and whose work is representative of nobility and monarchy....Superman, though a godlike alien, is closer to a democratic character.)
But let's look at the concept of the transformative transplant. Does someone with regeneration get used as a source of transplants? I remember reading something where the regenerator is essentially going to put himself or herself in the hospital forever so they can take lungs and spleens and whatever. But if you have regeneration, what kind of immune system do you have? Do the transplant recipients need huge quantities of immunosuppressants because the organ will just take over? Does the regenerating organ "know" when to stop? It's being nourished, so does it grow new other things, until the person pops open like rotten fruit and a clone of the donor steps out. (Which is pretty gross.)
Which might be your hook: someone wants that to happen, so they've intercepted the drug manufacturing for the immunosuppressants. Or replaced immunosuppressant B, which normally augments A, with something that suppresses A. And the clones start growing.
We can even throw in another SF trope which is that the clones, like identical twins, have some kind of psychic connection. The donor can control them or even flip between bodies. You do this once so there are thirty or a hundred of the regenerating villain around, and the donor can move between them or summon them, so the character never officially dies.