Barbarian-Wielding Barbarian

Some barbarians are smaller than others, and thus decide to pair up with larger beings so as to enhance both individuals' combat prowess. The larger warrior takes up the smaller as their weapon, with the damage coming from whatever "real" weapons the smaller barbarian has in hand. These twinned warriors are feared for their unpredictability above all - having a "living weapon" allows the duo to conduct some wild maneuvers in battle.

Boon of the Living Weapon

When you choose this subclass at 3rd level, this feature determines the basics of the relationship between the larger and smaller beings in this setup.

"You," your character's base race, serves as the "larger" barbarian in this subclass, and must be at least a Medium-sized creature or larger. Your "living weapon," the smaller barbarian, can be of any race Small-sized or smaller. You may choose one racial feature from the smaller race to add to your own - for instance, if this is an Orc wielding a Goblin, the Orc can now use any one of the Goblin's normal racial features.

The smaller barbarian - your living weapon - possesses HP equal to (your Barbarian level + your CON modifier + your proficiency bonus). If You die, the living weapon dies as well. The living weapon can only lose HP if You are not wielding it. If the living weapon is knocked out, it must be picked up and wielded by You by the start of your next turn or it must succeed a death saving throw. A success returns the living weapon to 1 HP. If it fails the death save, the living weapon is dead. You will have to wield normal weapons until you find another willing living weapon. The living weapon regains any lost HP after a short or long rest.

When wielding your living weapon, it counts as a Heavy Two-Handed martial weapon with the Reach property. The damage type you deal is based on the weapon being used by your living weapon - if the living weapon is using a greataxe, you'll deal the 1d12 + your STR slashing damage a greataxe normally deals. If your living weapon is unarmed, treat it as if you're using a normal Greatclub (WITHOUT the Reach property!) - the living weapon will punch or headbutt whatever you are aiming at for bludgeoning damage.

Your living weapon can only use a single one-handed or two-handed melee weapon; it cannot dual wield weapons OR use ranged weapons.

A Warrior of Two Minds

Starting when you choose this subclass at 3rd level, as a bonus action, you can choose to allow the living weapon to act independently.

If in combat, it then makes an Initiative roll and is added to the turn order. Once its turn begins, the living weapon can make a Move and an Action, but must remain within 30 feet of you. The living weapon has a movement speed of 30 feet and uses your ability scores and proficiency modifier when making its Action. 

While it cannot use ranged weapons when being wielded normally, the living weapon may use ranged weapons when acting independently.

While the living weapon is not being wielded, you may draw another weapon and finish your turn as normal. 

After its turn is complete, the living weapon returns to you to be wielded once again.

If NOT in combat, treat the living weapon as a separate character that uses your ability scores and proficiency modifier when conducting movement and actions. The living weapon can be separate from you until you want it to return to you, but it cannot be further than 120 feet from you.

Fastball Special

Starting at 6th level, your living weapon gains the Thrown (20/60) and Return properties, allowing you to make ranged attacks with it. Due to the extra mass of the living weapon, you may add an additional 1d8 to the damage roll. After the attack is resolved, the living weapon returns to you to be wielded once again.

Spin to Win

Starting at 10th level, you finally have mastered your living weapon enough to be able to temporarily use them one-handed. As an Attack action, you can draw another one-handed/versatile weapon and spin in a circle, using your living weapon and other weapon to hit all enemies in a 360-degree arc around you (remember, your living weapon also probably has Reach!).

Make the attack roll and count it against the AC of every individual enemy you could possibly hit - if you could hit five enemies, one attack roll is measured against each foe's AC individually. Example - if your attack roll is a 17, you hit every enemy in range with 16 or less AC.

Damage dealt equals what your living weapon can do plus what your secondary weapon can do (example: living weapon is a greataxe, secondary a longsword; total damage is 1d12+1d8+your STR modifier to each enemy hit).

Once the Attack is resolved, you must sheathe the secondary weapon and return to using your living weapon two-handed.

Unwilling Reload

Starting at 14th level, if you are temporarily separated from your living weapon or your living weapon has been slain, you can choose a new one from the enemies or innocent bystanders around you.

As a bonus action, you may conduct a Grapple check against a target creature within 5 feet of You that is at least one size smaller than You. If you win the Grapple, you now wield the unwilling creature as your weapon. The creature can re-roll their Escape from Grapple at the start of their next turn. The first time it re-rolls, its does so with disadvantage, but they then can roll normally for every turn afterwards.

If you Throw your unwilling weapon via a Fastball Special, it deals an additional 1d8 bludgeoning damage per 10 feet thrown on a hit, rather than the normal flat additional 1d8 dealt by the Fastball Special. In addition to damaging another enemy with the thrown unwilling weapon, the unwilling weapon takes the additional bludgeoning damage as well, and it does not Return to you afterwards.