How would you go about homebrewing appendage assimilation?

[Greywander]

I'm working on a homebrew feat that allows appendage assimilation. Also works for heads and torsos, though, so more like "body part assimilation". I'm trying to figure out a way to make it useful but neither overpowered nor too restrictive.

Speaking both generally and broadly, the purpose of this ability seems like it would be geared toward "borrowing" another creature's abilities. For example, graft a dragon arm onto your arm stump and that arm is as strong as the dragon it came from, or taking a couatl's eye would give you truesight. There's also the possibility of grafting extra body parts (adding wings to yourself is the first thing that comes to mind).

As far as limitations, creature size seems like an obvious one (let me point out that krakens also have truesight; imagine a gnome with kraken eyes). There's also the number of body parts that you can graft, though I'm not sure how it should scale or if it should include replaced body parts or just extra ones.

How would you go about implementing such an ability?

[Vogie]

It depends on the system.

In Pathfinder: Horror Adventures there's a Fleshcrafting mechanic you can sample from. They have options like Burrowing Claws, Darksight Eyes, Poison Fangs, Stinging Tail, and so on. Porting that over to whatever your system is would probably be best

If you have access to White Wolf materials, they've played around in this zone -

• Hunter: The Vigil has a group (a conspiracy, I believe) that grafts monster parts on their members to give them powers and a combat edge.
• Geist: the Sin-Eaters has a player choice archetype called Caul (IIRC) that alters the PC's body, with the Industrial Caul specifically including rules on how to store and integrate weapons and other things inside the PC's body
• Promethean: The Created are all about created beings, and the Wretched (Frankenstein-style) have this ability.

In 5e, the Undying Warlocks have a patron feature that allows them to reattatch severed limbs... it wouldn't be a stretch to allow them to use that feature to Hot-swap their arm for a Minotaur arm, for example, after dispatching one, and using that ability to give themselves an upgrade based on the new limb - in our minotaur example, it'd give a strength boost or a the Reckless attack feature, or something similar.

[BerzerkerUnit]

I would probably make this an Artificer archetype. Tier out what can be swapped out, possibly using monster CR or creature types as the baseline and the old Chakra Centers from the Soulmelders of 3.5. Then allow the feat to give you a minor option.

Been a while since I made an Artificer subclass but if there are 4 features, here you go

Fleshcrafter 1 Nightmare Surgeon
replace parts, these parts gradually lose their potency, features cease functioning after Con modifier days (can maintain with downtime for others, short rest for you) .can replace hands and feet with monster parts claiming claw attacks, climb and burrow speeds, or single uses of esoteric abilities, like a healing touch (you just cut paste exactly what's in the creature's entry and it becomes a Short Rest ability for you (or a rechargeable 1 off for allies). You need a complete set, so you can't have a Bullette's back paw for a burrow and a bear's back paw for climb.

Can adapt Beast and Monstrosity Parts

FC 3 You have your mother's eyes. Literally.
You can replace sensory organs granting the recipient enhanced senses or Gaze attacks or eye rays.

Can now adapt Fiend and Celestial Parts (cut off an Angel's hands for a healing touch).

FC 2 Did I use too many monkeys
Add parts. Adding a part grants advantage on certain kinds of checks or grants previously unavailable movement modes Wings for fly=walk speed. You can add up to Con bonus parts.

Can now adapt Aberration and Plant Parts

FC 4 Inside out Tin Man
You can now replace heart and throat claiming regeneration, agelessness, breath weapons, poisoned bites

Can adapt Construct and Elemental Parts

OMG I want to do a full write up of this and have an Aez (Ooze player race on the DM's Guild) that tries to take a more humanoid form... It would move like Octodad.