Subclass based on replacing body parts with powerful magic prosthetics. My first homebrew so let me know if too op or underpowered.
#Prosthetist (Artificer)
thick ocean
Arm (Level 3):
At 3rd level you replace one of your arms with a battlefist, a magical prosthetic you created. Your battlefist is a simple melee weapon, and you can use it as a spellcasting focus for your artificer spells. While your battlefist is a magical weapon, it can be infused as if it were a nonmagical weapon. (Requires 100gp worth of engine materials)
When you attack with your battlefist, you can use your Intelligence modifier instead of Strength for the attack roll. If you hit with it, you deal bludgeoning damage equal to 1d10 + your Strength or Intelligence modifier. At 9th level, the damage dealt by your battlefist increases to 2d10 + your Strength or Intelligence modifier.
Additionally, you can choose one of the following weapon properties to add to your battlefist: finesse, thrown (range 20/60), or reach. When you finish a long rest, you can replace this property with a different weapon property from the list.
You gain the following spells:
3: Thunderous Smite
5: Enhance Ability
9: Blinding Smite
13: Stone Shape
17: Banishing Smite
Eyes (Level 5):
At 5th level you replace both eyes with magic crystalline prosthetic eyes. (Requires two diamonds each worth 100gp)
You have 90ft of darkvision, and advantage against attack rolls or spells that would blind you.
You learn two languages of your choice, you can cast Comprehend Languages at will, and the eyes can cast the Light cantrip at will.
You gain the following spells:
5: Locate Object
9: Clairvoyance
13: Locate Monster
17: Telekinesis
Legs (Level 9):
At 9th level you replace both legs with metal legs with fitted engines. (Requires 500gp worth of engine materials). You gain 10 feet of walking speed, and you can jump a number of feet equal to five times your proficiency bonus.
You gain the following spells:
9: Misty Step
13: Freedom of Movement
17: Destructive Wave
Extra Attack (Level 15):
You can attack twice, rather than once, whenever you take the Attack action on your turn.
frozen atlas
First of all, all Artificer subclasses gain an extended spell list of 2 per level. Idk what youre thinking giving it as part of the main feature (it should be its own feature) and splitting it with the 5th level and 9th level feature
Second, if youre gonna give Extra Attack, its at 5th and nowhere else. All Artificer 5ths are simple self damage increases: Extra Attack for the weapon using artis and a way to add to spells for the caster focused ones (alch adds mod to all fire or acid or healing, artillerist adds d8 to damage)
Third, you lack a Tools of the Trade feature that gives a tool proficiency which is used by the Artificer for its invention (the main feature)
frozen atlas
Now...
Arm is fine, if a bit simplistic. Theres no reason to have the 100 gp cost, youre usually infusing items the party already has so why cost you to infuse? I realized this is the cost to make the prosthetic, but honestly I still think its near pointless. Should be a replacement cost if you lose or break it, but you get your first free.
Theres also almost no reason to have the Reach property, you can probably do Returning Thrown instead. Also no reason to use Finesse, since youre def using Int, unless youre Defensive Duelist I guess?
The damage buff can stay, pushed to 15th. Ideally it should be part of the 15th level feature imo, its a big bump.
Other than that... this level is quite lacking I think. Look at Armorer: dealing either d8 or d6 damage but has great extra riders and another feature like the defense field or stealth adv
As mentioned you need two features here: Expanded Spell List and Tools of the Trade. Cut down your spell list to 2 per level and throw it here.
Eye should be pushed to 9th. Its also nonsense to say "advantage against attack rolls" since youre not rolling them, the enemy is rolling them vs your AC. You should probably say you "have advantage on saving throws against effects that blind you".
Learning two languages here is far too late. Comprehend Languages is fine, but its really weird that EYES grants this. You should just say "You can read all writing" or something like the Eyes of the Rune Keeper invocation (I think thats the name?) Light cantrip is fine, but also really late. Plus you already have Darkvision.
Speaking of Darkvision, its kinda at an awkward range. 90 is not a usual range, its used for some spells but its not usual for darkvision. Its usually 60 and increments of it. Not a dealbreaker, just wanted to mention it.
Also as mentioned, push spells down to 3rd. You can keep the theming as unlocking it on those levels by tying the sight stuff correctly to 5th and above
Legs should be pushed to 15th. 10 feet walking speed is nice. Jump 5x prof is not bad, but usually not useful. This is underwhelming for 9th and even moreso for 15th, and the only other thing you're getting here is the damage boost of the Arm if you push it to 15 like recommended. Thats still pretty underwhelming.