Behemoth

Behemoths reject forged or crafted weaponry in favor of using their hands or head to shatter whatever threatens them.  Emulating the battle method of their chosen spirit animal, they focus on unleashing the fury of an apex herbivore upon their foes without regard for their own well being.  If struck down, they will stubbornly rise again to deliver another crushing blow, caring not for the effect it will have on their tomorrow.

Unbreakable Will

Rise from the ground, angered and unbreakable.
Starting at level 3, when you would make a death saving throw you may instead gain 1hp, rise from prone, and gain 1 level of exhaustion.
When activated, if you are level 9 or higher in this class, your AC increases by 2 until the end of your next turn. This effect stacks with itself.
When activated, if you are level 13 or higher in this class, until the end of your next turn you grow one size category larger until the end of your next turn. If there is no room to grow one size larger, you gain all the benefits of growing a size without actually getting larger. Your unarmed damage dice double as they would if attacking with a larger weapon. This effect stacks with itself. 

Stomp and Gore

Your primal urges tell you that weakness is needing any weapon except your body to destroy your foes.  At level 3 you gain the ability to imitate your chosen spirit animal by choosing bludgeoning, piercing, or slashing damage. From here on, you may perform an unarmed strike of the chosen type to deal 1d8 damage of the chosen type.

Unarmed-Bludgeoning

You smash your attacker with blunt force, breaking them beneath your superior strength.

Unarmed-Piercing

You drive a point through your attacker, leaving them skewered and bleeding.

Unarmed-Slashing

You tear furrows in your attacker's skin leaving them shredded.

Trample

Stomp them when they're down.  Starting at level 7, when a creature falls prone within 15 feet of you, you may move up to your movement speed and make a single natural weapon attack against that creature as a reaction.  If that attack hits, roll an additional 1d6 bludgeoning damage when calculating that attack's damage.

Square Up

Starting at level 7, as a bonus action, you may lock eyes with one creature, then turn your bulk square with their direction.  The creature must make a wis save against 10 plus your Constitution modifier.  You may subtract your Charisma score from the save DC as well.  On a failed save the creature gets the frightened condition and cannot move nearer to you.  The creature can make an additional save at the end of each of its turns until it saves or three turns have gone by, after which the effect fades.

You may use this ability a number of times equal to your proficiency bonus.

Rampage

Starting at level 11, you may use a bonus action to become more bestial in appearance for one minute, looking more like your spirit animal.  While in this state you roll an additional 1d6 bludgeoning damage on each unarmed strike attack.  You may use this ability twice per long rest.
Also, you've determined your dominant trait.  The body part that you use for your Stomp and Gore ability grows 25% permanently.  Your unarmed strike attacks count as magical.

Regent of the Land

As regent of the land, you stand unflinching when harmed, and your presence is felt from a distance by those that would prey on the weak.
Starting at level 11, you are proficient in constitution saving throws.  In addition, your presence causes predators of CR 1/2 and lower to vacate the area within a one mile radius of you.

Territory

Your knowledge of your terrain is unparalleled, as is your ability to fight within it.  Starting at level 15, if you fight within your preferred terrain, you may roll an additional 1d6 of your natural weapon's damage type on a hit.

Adrenaline Surge

The surprise of a new battle is a welcome pick-me-up. Starting at level 15, you may choose to lose one level of exhaustion at the beginning of any combat encounter. If you do so, you must make a DC10 wisdom save. On a failed save you must make one attack against the creature nearest you each round. If more than one creature is the same distance from you, choose randomly which creature to attack. You may make a new saving throw at the end of each of your turns, ending this effect on a successful save.