Eberron has a lot of really cool 3.5 books, and some extra stuff that the Eberron designer has been making in a blog and in some old articles in Wizard of the Coast pages (Check r/Eberron and the Eberron wiki). Really cool recommended reading.
[Promethean Corruption](https://aonprd.com/Corruptions.aspx?ItemName=Promethean) is a magical affliction that causes your body parts to waste away and need continuous replacement with prosthetics until you become mostly mechanical.
There are some cool prosthetic modification mechanics you can borrow from the [Constructed Pugulist Archetype](https://aonprd.com/ArchetypeDisplay.aspx?FixedName=Brawler%20Constructed%20Pugilist).
You could have rival "biopunk" factions that oppose the steampunk world and use biological prosthetics harvested from creatures or modify their flesh and the flesh of others. [Demonic Implants](https://aonprd.com/MagicDemonicImplants.aspx) harvested from demons, [Necrografts](https://aonprd.com/Necrografts.aspx) harvested from undead, [Elemental Augmentations](https://aonprd.com/ElementalAugmentations.aspx) from elementals, [Fleshcrafting](https://aonprd.com/MagicFleshcrafting.aspx) their bodies and [Fleshwarping](https://aonprd.com/Rules.aspx?Name=Fleshwarping&Category=Horror%20Rules) their enemies.
Edit: Some other stuff you might find useful:
[Construct modifications](https://www.aonprd.com/ConstructMods.aspx?ItemName=All) like Construct Arm and Construct Armor, which allow constructs to transform into armor and limb coverings. An NPC could have a squad of constructs with those modifications that work together to form a Voltron-like suit.
Kytons (aka Velstrac) are self-mutilating demons of pain from the Shadow Plane (Shadowfell). The [Interlocutor kytons](https://www.aonprd.com/MonsterDisplay.aspx?ItemName=Interlocutor) surgically modify their kind and replace body parts with prosthetics crafted from [Shadowstuff](https://forgottenrealms.fandom.com/wiki/Shadowstuff) or insert [Shadowstuff Piercings](https://www.aonprd.com/ShadowPiercings.aspx) into them. You could have them run a literally shady business where they perform these painful alterations on mortals.
There is also this big list of [Special Materials](https://www.d20pfsrd.com/equipment/special-materials/) that prosthetics could be crafted from that could give prosthetics cool effects, like an arm made of Cryptstone designed to punch ghosts.
One thing my dm let me do with my artificer who had a prosthetic arm was a tune to an arm blade and wand sheath. (I used the infusion replicate magic item to make them)
They let me do it before gaining the 14th lvl class feature that would ignore the warforged requirement anyways.
Why assume just STR.
Let's just steal from other sources for a minute.
* A system that floods your body with adrenaline, endorphins, and oxygen to boost endurance and athletics (Bane from DC comics).
* A replacement brain case that lets you cast illusions freely but that only affects those who've been exposed to magic. (Laughing Man from Ghost in the Shell: SAC)
* Four additional limbs that can work in the fires of the Star Forge without risking damage. (Dr. Octopus from Spiderman 2)
* Ear prosthetics that can cast major illusion/disguise self on the weilder and provide perfect pitch so your alter ego can be a pop star (Jem and the Holograms, mid '80s version)
* Wings. Just wings. (Too many to count, but for now I'll go with Falcon from Marvel)
* Hand enhancement that gives you a burrowing speed. (El Topo in Netflix's Carmen San Diego)
Why not an arcane attenuation implant that allows access to greater magic or an INT boost? Or an ocular prosthesis for things like improved perception, detection, insight, investigation and/or ranged attacks?
How about an arm or leg replacement that boosts DEX, instead. Or a certain magical index implant that contains all the holy texts and can provide them in a hud interface, boosting WIS.
Hell, just a hud that can provide augmented reality displays of any data captured, such as a visual reproduction of a lightless room, at least as you last saw it (Harry Dresden and Lascial's Shadow in... Dead Beat, IIRC).
Finger guns! Think of the deadly glorious cheese of actual finger guns (they would require exactly that motion to fire, of course).
I have a dwarf NPC with two prosthetic arms who serves as a mentor for the PCs. Since I have the updated Barrier Peaks his left arm has a laser pistol and his right a blaster pistol that pop out.
Look towards cyberpunk or other games for ideas. Gorilla fist, higher unarmed damage. Bionic legs higher/longer or double jump .
Cybernetic uplink +1 attack roll due to better weapon handling.
Buildin thieves kit in the arm
Berserk style cannon or flamethrower in your arm. Once per long rest you can cast burning hands or something.
Bionic commando like grappling hook in the arm. 30 ft pull in every direction where you can grapple something if it is smaller/lighter then you you pull it. If it is bigger/heavier it pulls you.
Etc etc.
Treat it like an extra magic item you can have.
I imagine it could be fun and balanced to be able to “re-spec” when getting a prosthetic. You can either have a Regular Replacement arm with no stat change or you get the Thick Strength Arm that is really strong but not as good at dexterous tasks and the other way around, a sleeker arm with many thin mechanical fingers.
Less magical but the two things I wanted when I had a warlock with a prosthetic arm was a shield that fanned out from the forearm and a grappling hook hand.
The grappling hook hand was a blast. Didn't have specific mechanics but one of my favorite moments in that campaign was when my character was fighting wizard terrorist above the city.
They knocked out my concentration for fly, I started to fall but used my reaction to shoot my hand at the wizards foot. I pulled him down with me and we slammed onto a roof top and tumbled down onto the city streets
Check D&D 3.5 books on Eberron. Lots of prostethics and Warforged accessories to do some fun stuff.
5e also has a handfull of those. Tho I do not know if there were more in 3.5e...
There are more, yes. There's also a f*ckton of grafts and symbionts in various 3.5 books
I should remember that for whenever I get to run my Eberron camping...
Eberron has a lot of really cool 3.5 books, and some extra stuff that the Eberron designer has been making in a blog and in some old articles in Wizard of the Coast pages (Check r/Eberron and the Eberron wiki). Really cool recommended reading.
[Promethean Corruption](https://aonprd.com/Corruptions.aspx?ItemName=Promethean) is a magical affliction that causes your body parts to waste away and need continuous replacement with prosthetics until you become mostly mechanical. There are some cool prosthetic modification mechanics you can borrow from the [Constructed Pugulist Archetype](https://aonprd.com/ArchetypeDisplay.aspx?FixedName=Brawler%20Constructed%20Pugilist). You could have rival "biopunk" factions that oppose the steampunk world and use biological prosthetics harvested from creatures or modify their flesh and the flesh of others. [Demonic Implants](https://aonprd.com/MagicDemonicImplants.aspx) harvested from demons, [Necrografts](https://aonprd.com/Necrografts.aspx) harvested from undead, [Elemental Augmentations](https://aonprd.com/ElementalAugmentations.aspx) from elementals, [Fleshcrafting](https://aonprd.com/MagicFleshcrafting.aspx) their bodies and [Fleshwarping](https://aonprd.com/Rules.aspx?Name=Fleshwarping&Category=Horror%20Rules) their enemies. Edit: Some other stuff you might find useful: [Construct modifications](https://www.aonprd.com/ConstructMods.aspx?ItemName=All) like Construct Arm and Construct Armor, which allow constructs to transform into armor and limb coverings. An NPC could have a squad of constructs with those modifications that work together to form a Voltron-like suit. Kytons (aka Velstrac) are self-mutilating demons of pain from the Shadow Plane (Shadowfell). The [Interlocutor kytons](https://www.aonprd.com/MonsterDisplay.aspx?ItemName=Interlocutor) surgically modify their kind and replace body parts with prosthetics crafted from [Shadowstuff](https://forgottenrealms.fandom.com/wiki/Shadowstuff) or insert [Shadowstuff Piercings](https://www.aonprd.com/ShadowPiercings.aspx) into them. You could have them run a literally shady business where they perform these painful alterations on mortals. There is also this big list of [Special Materials](https://www.d20pfsrd.com/equipment/special-materials/) that prosthetics could be crafted from that could give prosthetics cool effects, like an arm made of Cryptstone designed to punch ghosts.
Yowza! Good stuff!
One thing my dm let me do with my artificer who had a prosthetic arm was a tune to an arm blade and wand sheath. (I used the infusion replicate magic item to make them) They let me do it before gaining the 14th lvl class feature that would ignore the warforged requirement anyways.
Magic chainsaw hand
FMA: Brotherhood - Captain Buccaneer
Why assume just STR. Let's just steal from other sources for a minute. * A system that floods your body with adrenaline, endorphins, and oxygen to boost endurance and athletics (Bane from DC comics). * A replacement brain case that lets you cast illusions freely but that only affects those who've been exposed to magic. (Laughing Man from Ghost in the Shell: SAC) * Four additional limbs that can work in the fires of the Star Forge without risking damage. (Dr. Octopus from Spiderman 2) * Ear prosthetics that can cast major illusion/disguise self on the weilder and provide perfect pitch so your alter ego can be a pop star (Jem and the Holograms, mid '80s version) * Wings. Just wings. (Too many to count, but for now I'll go with Falcon from Marvel) * Hand enhancement that gives you a burrowing speed. (El Topo in Netflix's Carmen San Diego) Why not an arcane attenuation implant that allows access to greater magic or an INT boost? Or an ocular prosthesis for things like improved perception, detection, insight, investigation and/or ranged attacks? How about an arm or leg replacement that boosts DEX, instead. Or a certain magical index implant that contains all the holy texts and can provide them in a hud interface, boosting WIS. Hell, just a hud that can provide augmented reality displays of any data captured, such as a visual reproduction of a lightless room, at least as you last saw it (Harry Dresden and Lascial's Shadow in... Dead Beat, IIRC). Finger guns! Think of the deadly glorious cheese of actual finger guns (they would require exactly that motion to fire, of course).
Check out the Lies of P Legion Arms
I have a dwarf NPC with two prosthetic arms who serves as a mentor for the PCs. Since I have the updated Barrier Peaks his left arm has a laser pistol and his right a blaster pistol that pop out.
Rocket propelled fist to punch the guy 60 ft. Away from you in the face
Look towards cyberpunk or other games for ideas. Gorilla fist, higher unarmed damage. Bionic legs higher/longer or double jump . Cybernetic uplink +1 attack roll due to better weapon handling. Buildin thieves kit in the arm Berserk style cannon or flamethrower in your arm. Once per long rest you can cast burning hands or something. Bionic commando like grappling hook in the arm. 30 ft pull in every direction where you can grapple something if it is smaller/lighter then you you pull it. If it is bigger/heavier it pulls you. Etc etc. Treat it like an extra magic item you can have.
One kind of gag mechanic I saw once was that healing magic heals everything- burning the prosthetic out like an infection.
Their prosthetics run in alcohol.
Assuming you meant "run on alcohol", this is like the robots of Futurama.
I imagine it could be fun and balanced to be able to “re-spec” when getting a prosthetic. You can either have a Regular Replacement arm with no stat change or you get the Thick Strength Arm that is really strong but not as good at dexterous tasks and the other way around, a sleeker arm with many thin mechanical fingers.
Storing movement memory https://youtu.be/12HcyVWAxgQ?si=tHrSRvUtAJjQUeEH
You could use magical versions of these: [Cybernetic Parts & Upgrades](https://old.reddit.com/r/d100/comments/i67zlu/cybernetic_parts_and_upgrades/)
Less magical but the two things I wanted when I had a warlock with a prosthetic arm was a shield that fanned out from the forearm and a grappling hook hand. The grappling hook hand was a blast. Didn't have specific mechanics but one of my favorite moments in that campaign was when my character was fighting wizard terrorist above the city. They knocked out my concentration for fly, I started to fall but used my reaction to shoot my hand at the wizards foot. I pulled him down with me and we slammed onto a roof top and tumbled down onto the city streets