I've actually been thinking about this a fair bit.
It wouldn't be pure perplexicon because, well, if I gave you a fire pancreas what the hell would you do with it?
My idea of how it would work is like this:
You'd have 5 "lines", your arms, legs and body. So Upper arm+lower arm+hand, Upper leg+lower leg+foot, and Gut+chest+head. Combinations of these parts make up the end effect, and the more distal the part, the greater the effect.
Lets look specifically at the arm.
Arm has the Hand, the forearm and the upper arm. Hand is most distal so it has the greatest influence on the over all end effect. Forearm is middle, so it would determine more the nature or makeup of the effect, and the upper arm would give an attribute or passive additional effect.
Now, assume we have the arm of a Flesh Monger, a type of animate skeleton that tears the flesh off of living things to affix on its bones and make itself stronger. This critter has the attributes "Drain" "Flesh" and "Parasite"
If I take the creature's hand and replace my own with it, it gives me the ability to do a drain attack, dealing continuous damage so long as I keep hold of the enemy.
If I take the forearm, it gives me the element of flesh. On its own this doesn't really do anything, but combined with other hand abilities it will use or give mastery over flesh. If I were to take the hand and forearm of the Flesh Monger, it would give me a drain attack that specifically tore away at flesh, dealing a lot more damage to organic things.
If I take the upper arm, it has the attribute "Parasite", which again does nothing on its own, but it will make attacks using that arm parasitic, restoring me as I hit with them. And this can include just normal melee attacks. And if I combine ALL the parts, taking the whole arm, I get an effect where I can grab a living thing, drain the flesh off it, and restore myself by doing so.
Aside from these powers, parts can also confer general stat bonuses. So, if I didn't like any of those abilities that the arm had, I could have just taken the parts and slotted them as "Stat" parts, giving me +1's in various stats. Or more realistically, +1/3rds or something, since each part giving +1 might become rapidly OP.
ALSO, This system allows for leveling up via cannibalism. Specifically, in order to level up a stat, you have to eat enough parts that have that stat as their "Boost", ie the stat that would be added to if you used them on your own body.
Replace the "Faith" stat with "Humanity", have it drain humanity to use active abilities, have you regain humanity points based on how many human parts you still have.
As people get deeper in and kill more creatures, it unlocks a book full of demon parts to choose at spawn instead of having to gather as you go though.
Be a lost soul in ORO! Murder your way through hell to try and escape! Cannibalize demons! Steal their parts! Just make sure not to become a demon in the process!