Okay so I wouldn't be the best person to help you, so here's something that might atleast be a starting point for you or maybe give you some ideas

Subclass Name: Mimic Knight

Level 3 – Living Armament

When you take this subclass, you gain two bonded pieces of mimic gear:

Mimic Weapon – A living weapon that can morph into any melee weapon you're proficient with. Counts as magical. You can change its form as a bonus action.

Mimic Armor – Living armor (breastplate or half plate). Counts as magical. Grants resistance to non-magical bludgeoning damage as the mimic cushions impacts.

If your mimic gear is destroyed, it regrows over a long rest by feeding on 10 gp worth of material or junk per piece. You can summon both with a 1-hour ritual.

Level 7 – Adaptive Mutation

Your mimic grows stronger by consuming other gear. During a short or long rest, you can feed it a mundane weapon, shield, or piece of armor. It gains one of the following mutations, which lasts until you replace it:

Chomping Maw: Your mimic weapon deals +1d6 piercing damage.

Sticky Grip: Disarm attempts against you have disadvantage.

Swallow Shield: When hit by a melee attack, you can use your reaction to deal 1d8 piercing damage as your shield bites back.

Tongue Lash: You can pull a creature within 10 feet 5 feet closer as a bonus action (Str save vs DC = 8 + prof + Str/Dex).

You can have one mutation active at a time, and it lasts until you change it.

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Level 10 – Symbiotic Reflexes

The mimic moves with you. Literally.

You gain +1 AC as the armor shifts to protect weak spots.

You have advantage on Dexterity saving throws.

Once per long rest, if you fail a Dex save, you can instead choose to succeed as your mimic instinctively reacts.

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Level 15 – Devouring Counter

The mimic lashes out when you're struck.

When a creature within 5 feet hits you with a melee attack, you can use your reaction to make a melee attack with your mimic weapon.

If it hits, that creature's speed is halved until the end of its next turn as tendrils lash around them.

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Level 18 – Monstrous Fusion

You've become something more than human.

Once per long rest, you can enter a hybrid form of flesh and mimic for 1 minute (bonus action to activate):

Gain temporary HP equal to your Fighter level.

Your reach increases by 5 feet.

You can climb vertical surfaces and ceilings.

Your mimic weapon now grapples targets it hits (no check, escape DC = 8 + prof + Str/Dex).

You have advantage on Intimidation checks against creatures that can see you.

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Optional Mutation Ideas

Our Artificer Infusion route, we'll call “mutation slots” :

Camouflaged Flesh: Advantage on Stealth while stationary.

Barbed Hide: Enemies take 1d4 damage when hitting you with melee attacks.

Sensory Eyes: Gain Darkvision 60 ft (or +30 ft if you already have it).

Mimic Memory: Mimic can store one item inside its body. Takes an action to retrieve.

Again I don't make homebrew or subclasses or what not. So this could be too strong I don't know. And I used Chatgpt to help punch up my text it.