Beast of Battle

Unlike other more refined fighters, you are a self-taught monster that can wield anything as a weapon. Your only practical training involved putting your life on the line over and over, because of that, you can not only adapt faster, but also learn quicker than other fighters. 

Level 3: Traumatized to Improvise

By level 3, you have learned to use anything and everything as a weapon. Tools and items that were not considered weapons are now considered Improvised Weapons for you, and you are Proficient in wielding them. These tools and items have different effects and uses that vary greatly between each item. When you are using an item or tool that can reasonably do something, like a rope that can be used to grapple or a Crowbar that can be used to Disarm or knock an enemy Prone, the save DC will be (8 + Strength modifier + Proficiency Bonus). Interactions with enemies using Improvised weapons from the field add +2 to their Save DC, the first time they are used, due to being unpredictable. The damage type of your improvised weapons varies based on the item or tool, but the damage of your improvised weapons is as follows:
Thrown: 1d4
Body: 1d6
One-Handed: 1d8
Two-handed: 1d12

How you use an item determines if it is One-Handed or Two-Handed (IE. Using a Rope as a Whip is One-Handed, While using it to choke someone is Two-Handed)

Level 3: Built for Battle

Starting at level 3, You have learned how to better perceive the teachings of other fighters in order to immediately add it into your own unique fighting style. Fighter techniques you learn use half your Proficiency modifier immediately, when calculating Damage, Hit modifiers, and Save DC. You also cut the amount of times you need to successfully use the technique before Mastering it in half.

Level 3: My Body is a Weapon too

Starting at level 3, After hitting an enemy twice with any weapon, you may make a third attack against them as a part of your Action/Bonus Action (whichever you last used to attack) using your body. This third attack uses the same modifier To-Hit but does less damage. You may choose to do one of the following to any creature that is your size or smaller. Larger creatures have advantage against these effects and smaller creatures have disadvantage:

Headbutt: 

Deals 1d6 bludgeoning damage to a creature and 1d4 bludgeoning damage to yourself. The creature must succeed on a Constitution save or be Stunned until the start of their turn.

If the creature was Concentrating on a spell, their roll to maintain concentration is twice the damage roll.

At level 10, Headbutt does 1d12 bludgeoning damage to creatures you use it on, and you take no damage when it is used against creatures made of Flesh.

Leg Stomp:

Deal 1d4 bludgeoning damage and attempt to cripple the enemy, halving their Speed for until the end of their next turn. They must roll a 1d20 at the end of their next turn, if they roll a 1, their speed remains halved until their next turn ends.

At level 10, Leg-stomp’s secondary effect triggers on a roll of 1, 2 or 3.

Tackle:

The creature must succeed on a Strength save or be knocked Prone and considered Grappled by you.

You are considered Prone for the sake of interaction between you and other creatures besides the one you tackled.

The creature must make a Strength save at the end of each of their turns, on success they are freed and you are no longer considered Prone.

At level 10, Tackle Restrains the creature you tackled as long as it is your size or smaller.

Kick:

Deal 1d6 bludgeoning damage. The creature must succeed on a Strength save or be knocked back 5 feet.(Creatures smaller than you get knocked back by 10 feet instead)

At level 10, Kick deals 2d6 bludgeoning damage and knocks the target back 10 feet. (Creatures smaller than you get knocked back by 20 feet instead)

Level 7: Your Minions are my Tools

Starting at level 7, You are able to use creatures Smaller than you as Improvised two-handed weapons. The creature must succeed on either a Dexterity or Strength save (Creatures choice) or be Restrained by you until the end of your turn. Both the creature being used and the one being hit take the same damage.

If the creature is used to hit the ground or field, roll your attacks with Advantage.

The small creature that was used is knocked Prone within 5 feet of you at the end of your turn, unless it was Thrown or Kicked away.

Level 10: Insidious Killer

By level 10, you have grown accustomed to taking advantage of the weaknesses you have inflicted on your enemies, and know how to extort them for greater gain. For every unique negative condition a creature you hit is affected by, your attacks gain +1 to Hit and the Save DC for your attacks additional effects increases by 1. This bonus resets at the start of each of your turns or when changing targets.

Level 15: Creative Combat

At level 15, For every different weapon you successfully hit a creature with consecutively, increase the damage of your next damaging weapon attack by 2. This ability and its damage resets when you hit a creature with a weapon you have already hit them with before.(Damage starts after the second weapon hits, the damage cap is equal to your Fighter level and ends after 1 turn of not attacking or after missing an attack on a creature.)

Level 18: Best in Battle

By level 18, your techniques and skills have begun to shine the brightest in the midst of bloody combat. Whenever you make a Skill check while in combat, you can add your proficiency modifier to the roll even if you are already proficient in that skill.