Hmm, how about creating a table for possible effects of organ replacements/transformations/transplants/implants (the exact effects would, of course, depend on the nature of the replacements/transformations/transplants/implants, but suggestions may be helpful or inspirational - I list only bonuses, but in the cases of some creatures these can be reversed and equivalent penalties created):-
Adrenal Glands:- replacement changes response to danger - possible gain of rage-like effects (thanks Mayhem!)
Bones or Skeletal Material:- replacement changes resistance to damage and flexibility - possible changes to hit points, possible increases in dexterity or even strength (stronger bones, or more flexible bones, or more pliant joints...)
Hair:- replacement changes resistance to cold - possible increase in the ability to endure cold and possible increase in AC (if the fur is particularly thick)
Kidneys:- replacement changes water retention and resistance to poisons - possible decrease in the need to drink water and possible bonus to second saving throws against poisons (they get flushed out faster)
Lungs:- replacement changes breathing - possible increase in the ability to hold breath, or possible gain of the ability to breathe water, or increased endurance
Muscles:- replacement changes strength - possible bonus to strength, or poossible increase in hit points
Nails:- replacement changes unarmed damage and/or improves ability to climb - possible bonus to climb checks, or bonus to unarmed attack, or bonus to unarmed damage (thanks Terwox!)
Reproductive Organs:- replacements change fertility and ability to conceive, carry a child and give birth - what it says above
Sensory Organs:- replacements change sensitivity of organs - possible gain of the Scent ability, or Low Light Vision, or Dark Vision, or Blindsight, or a bonus to Listen, Spot or Search skills
Skin:- replacements change appearance, camouflage and resistance to damage - possible increase in AC due to natural armour, or possible damage reduction (perhaps only against specific weapon category such as piercing), or possible bonus to hide checks due to camouflage pattern or the ability to change colour and texture
Stomach, Intestines and Other Digestive Organs:- replacement changes food intake and digestion - possible gain of resistance to poisons, or the ability to survive on different types of food depending on source creature, or the ability to survive on less food
Any comments and further suggestions would be appreciated!