My player is really wanting to lean into his Steel Defender and is envisioning a Battle Mecha. I know the Artificer is already really versatile and has tons of toys to play with, but I agree that once you've assigned those assets it can be a little frustrating with the limited choices you have. So, I wanted to work on some that would replace a feature from his class.

I am currently looking at replacing the Magical Tinkering Feature and giving the Steel Defender some modifications of equal 'power' or removing the Extra Attack from the Battle Smith and giving the Steel Defender some more powerful abilities. Below is what I have drafted up so far and am hoping for some feedback!!

Steel Defender Modifications
You have invented numerous mechanical and magical modifications for your Steel Defender that change it's functions and you store that knowledge in the form of blueprints. You know a number of blueprints equal to your Intelligence modifier (minimum of one) and can choose to throw away any number of blueprints and invent new ones when you gain a level in this class. You can have a maximum of two modifications on the Steel Defender with this feature at one time. When you reach 10th level in this class that number increase to a maximum of three and when you reach 14th level in this class that number increases to a maximum of four.

Some blueprints have prerequisites or costs which must be met/spent when the modification is installed. The blueprints used to create these modifications are magical in nature and cannot be destroyed by fire or water. If you lose your blueprints you can recreate it in a process that takes 1 hour per blueprint.

Whenever you finish a long rest, you can spend 1 hour modifying your Steel Defender with one of the Steel Defender Modifications, changing a primary function. Each modification after the first requires an additional 2 hours to perform. 

 

Battle Smith Blueprints
The items in the (parenthesis) are the cost of the modification.
Movement Speed Type:30ft Burrowing (Claws or another piece from a burrowing creature)
30ft Swimming (Scales or another piece from a swimming creature)
30ft Climbing (Claws or another piece from a climbing creature)
20ft Flying (Feathers or another piece from a flying creature)

Luminescence: As an Action the Steel Defender can become luminescent and sheds bright light in a 20-foot radius and dim light for an additional 20 feet. As an Action the Steel Defender can focus the light in a beam creating a line 10 feet wide and 30 feet long of bright light that sheds dim light for an additional 5 feet in all directions.

Damage Type:
Lightning (Topaz Gem 5gp Value, Consumed)
Fire (Ruby Gem 5gp Value, Consumed)
Cold (Sapphire Gem 5gp Value, Consumed)

Overcharge: When the Steel Defender hits with a melee attack you can choose to Overcharge the attack by using your Reaction and expending a spell slot of 1st level or higher. The Steel Defender adds an additional 1d8 damage per spell level spent to the attack to a maximum of 3d8.

Outriggers: As an Action the Steel Defender can deploy outriggers which stabilize it and prevent it from being knocked prone. While the outriggers are deployed in this fashion the Steel Defender's movement speed becomes 0ft, however, it is immune to the Prone Condition. The Steel Defender also has advantage on any Strength Checks it makes involving large weights (Using it as an anchor point when tying a rope to it as you climb up or down for example).

Protective Cover: As an Action the Steel Defender deploys a barrier that can act as cover for Medium or smaller creatures. The barrier is 10ft tall and extends 5ft on all sides of the Steel Defender.

Hidden Compartments: Installed into the Steel Defender are hidden compartments. When this modification is installed you pick the number and sizes of the compartments based on your Intelligence Modifier (a minimum of 1 Tiny). A tiny compartment can hold maximum 10 coins or items of similar sizes. A small compartment can hold a dagger or a potion sized item. A medium component can hold a helmet or a lantern sized item. These items still count towards the weight total of the Steel Defender. A creature can make an Intelligence (Investigation) Check versus your Spell Save DC to detect the compartment.
Tiny Compartments = 0.5 pts, Small Compartments = 1pt, Medium Compartments = 2pts

Self Destruction Switch: When the Steel Defender is reduced to 0hp it explodes in a 5ft blast of magical energy. Each creature within 5ft of the Steel Defender must make a Dexterity Saving Throw versus your Spell Save DC. On a failure the creature takes 1d8 + your Artificer Level of Force Damage.

Mighty Leap: The Steel Defender's long jump is up to 20 feet and its high jump is up to 10 feet, with or without a running start.

Propulsion Blast: As a Bonus Action the Steel Defender can take the Dash Action once per day.

(Any and all edits not mentioned are grammar corrections, spelling corrections, or added Modifications.)
(All major changes will be noted in the post.)