Barbarian

Source: Xanathar's Guide to Everything

Some barbarians hail from cultures that revere their ancestors. These tribes teach that the warriors of the past linger in the world as mighty spirits, who can guide and protect the living. When a barbarian who follows this path rages, the barbarian contacts the spirit world and calls on these guardian spirits for aid.
Barbarians who draw on their ancestral guardians can better fight to protect their tribes and their allies. In order to cement ties to their ancestral guardians, barbarians who follow this path cover themselves in elaborate tattoos that celebrate their ancestors’ deeds. These tattoos tell sagas of victories against terrible monsters and other fearsome rivals.

Source: Sword Coast Adventurer's Guide

Known as Kuldjargh (literally “axe idiot”) in Dwarvish, Battleragers originated from dwarven followers of the gods of war. They specialize in wearing bulky, spiked armour and throwing themselves into combat, striking with their body itself and giving themselves over to the fury of battle. Though early in history this Primal-Path was a closely guarded secret, Today this Primal-Path is avalible to all species.

Source: Tasha's Cauldron of Everything

Barbarians who walk the Path of the Beast draw their rage from a bestial spark burning within their souls. That beast bursts forth in the throes of rage, physically transforming the barbarian.
Such a barbarian might be inhabited by a primal spirit or be descended from shape-shifters. You can choose the origin of your feral might or determine it by rolling on the Origin of the Beast table.

Source: Player's Handbook

For some barbarians, rage is a means to an end – that end being violence. The Path of the Berserker is a path of untrammeled fury, slick with blood. As you enter the berserker's rage, you thrill in the chaos of battle, heedless of your own health or well-being.

2024 Revision Available

Source: Grim Hollow Pg 39

Barbarians are defined by their rage, channelling emotion to unleash brief but potent destruction on their enemies.
A rare few study esoteric psychological techniques that split their rage off from the rest of their psyche, dividing their identity into two parts: ego and id.
When their ego is in control 'the Fractured'; as these barbarians are known, are capable of a degree of self-control and cunning few other barbarians are. When they allow their id to take control, their countenance turns monstrous and their bodies swell with the power of rage made physically manifest

Player Story Tips & Ideas Players should consider some sort of split personality trait similar to Jekel & Hyde or the Hulk. Their rage form being a different personality to their regular selves.
Players should also consider how the character got this particular sub-class. experiment gone right or wrong? curse? blessing? etc.

What to Expect Players can expect to gain separate lines of features for when they are in rage and out or rage. The theme of Jekale & Hyde or Hulk. Gaining bonuses on Unarmed attacks rather than weapon attacks.

What would DM Darwin Change? As a Barbarian, you always want to be raging while in combat. so combat benefits you get while not raging seem kind of pointless.
I would change 'Brains & Brawn' to give non-rage an advantage on some social or exploration skill rather that Psy Resistance.
and changing 'Cunning & Brutal' to be able to give an alley some sort of social or Exploration bonus while non raging rather than being able to Disengage/Help as a bonus action when NOT raging.

Source: Bigby Presents: Glory of the Giants

Barbarians who walk the Path of the Giant draw strength from the same primal forces as giants. As they rage, these barbarians surge with elemental power and grow in size, taking on forms that evoke the glory of giants.
Some barbarians look like oversized versions of themselves, perhaps with a hint of elemental energy flaring in their eyes and around their weapons. Others transform more dramatically, taking on the appearance of an actual giant or a form similar to an Elemental, wreathed in fire, frost, or lightning.

Source: Guide to Drakenhime, Pg 150

The first warriors who ventured into Drakkenheim were doomed. Few returned from these early expeditions, and those few who survived were forever changed by the experience. A rare few wrestle the madness of the Haze through a supernatural rage which allows them to channel the eldritch energies of delerium. These Haze-Ragers thrive on the contaminated power of delerium, transforming madness and fear into a weapon. Their practice has given them unnatural tolerance for contamination, and some have told legends of these barbarians surviving even the most horrific mutations.

Source: Guide to Drakenhime, Pg 151

These barbarians consider themselves natural weapons, born of the gods to stand against the enemies of their chosen deity. The Old Gods represent raw power, the untamed wild, and traditional worship. A barbarian who follows the Old Gods is often close to nature, and sees glorious battle as the arena of their gods, and their purpose is to destroy the enemies of their chosen god.

Source: Grim Hollow Pg 40

Many barbarians are in tune with the natural world, but few are as mystically intertwined with it as those who walk the Path of the Primal Spirit.
These barbarians forge powerful connections with the beasts and natural spirits of the world, inspiring such ethereal beings to manifest and journey with them on far-flung adventures. Barbarians who follow this path have a deep reverence for the ecological cycle of the natural world. Such barbarians are as likely to accept quests and pleas for aid from local wildlife as they are other humanoids.
This respect for animals doesn’t cross into naivety—no barbarian better understands the circle of life and the delicate balance between the needs of predator and prey better than those who walk the Path of the Primal Spirit.

Player Story Tips & Ideas Players can fit any simple Hunter, Tracker, Wilderness explorer, lone-wolf ideas into this subclass. Anything that may fit a Ranger or Druid may also work here. Maybe the character is part of a Ranger conclave or a Slayer (Hunter of Monstrosities).

What to Expect Playing a Barbarian but you get a cool Beast Companion with a variety of forms that Rages alongside you, Gaining benefits with level ups.

What would DM Darwin Change? Let the player command the beast companion as part of entering a Rage as well as a Bonus action, This means the player dosent have to wait a turn to rage then next tur command their beast companion.
'Kin to Beasts'
Let spells be cast a number of times equal to half proficiency bonus, Gives the player more leyway to use these free spells rather than constantly saving their single use per rest.
add the ability to have two primal spirits at the same time and be able to command both with entering a rage & Bonus action. Whats better than one hunting companion? two.

Source: Xanathar's Guide to Everything

Typical barbarians harbour a fury that dwells within. Their rage grants them superior strength, durability, and speed. Barbarians who follow the Path of the Storm Herald learn instead to transform their rage into a mantle of primal magic that swirls around them. When in a fury, a barbarian of this path taps into nature to create powerful, magical effects.
Storm heralds are typically elite champions who train alongside druids, rangers, and others sworn to protect the natural realm. Other storm heralds hone their craft in elite lodges founded in regions wracked by storms, in the frozen reaches at the world’s end, or deep in the hottest deserts.

Also Known as Totem Warrior

Source: Player's Handbook

The Path of the Wild Heart is a spiritual journey, as the barbarian accepts a spirit animal as guide, protector, and inspiration. In battle, your spirit animal fills you with supernatural might, adding magical fuel to your barbarian rage.
Most barbarian tribes consider a spiritual animal to be kin to a particular clan. In such cases, it is unusual for an individual to have more than one animal spirit, though exceptions exist.

2024 Revision Available as Wild Heart

Source: Tasha's Cauldron of Everything

Many places in the multiverse abound with beauty, intense emotion, and rampant magic; the Feywild, the Upper Planes, and other realms of supernatural power radiate with such forces and can profoundly influence people.
As folk of deep feeling, barbarians are especially susceptible to these wild influences, with some barbarians being transformed by the magic. These magic-suffused barbarians walk the Path of Wild Magic. Elf, tiefling, aasimar, and genasi barbarians often seek this path, eager to manifest the otherworldly magic of their ancestors.

Source: Hexbound, Pg 49

Barbarians fill a strange role in the ways of the witch community. Instead of seeking renowned mentors, the first of them took their learnings and went deep into the wilds in search of ancient witchcraft.
Some are taught by old spirits, others by hags, and fewer still by the voices of the woods themselves. They grow an internal force of explosive witchcraft, and often roam the world to release their magic on whatever they see fit.

Source: 2024 Player's Handbook

Trace the Roots and Branches of the Multiverse. Barbarians who follow the Path of the World Tree connect with the cosmic tree Yggdrasil through their Rage. This tree grows among the Outer Planes, connecting them to each other and the Material Plane. These Barbarians draw on the tree’s magic for vitality and as a means of dimensional travel.

Source: Xanathar's Guide to Everything

Some deities inspire their followers to pitch themselves into a ferocious battle fury. These barbarians are zealots – warriors who channel their rage into powerful displays of divine power.
A variety of gods across the realm of Felwind and realms beyond inspire their followers to embrace this path. Hoar The Poet of Vengeance, Bane The Black Lord and Glaren the Lizard of Life are all prime examples. In general, the gods who inspire zealots are deities of combat, destruction, and violence. Not all are evil, but few are good.