Ok, so here's what I've come up with.
Fighting Style Mastery
You mastered your particular style of fighting. You gain the benefits below associated with your fighting style. If you have more than one fighting style, you may choose which mastery benefit you gain.
Peerless Archer
Requires the Archery fighting style
Your arrows always sail true and deal maximum damage. Once per round when you make a ranged attack, you can choose to have your attack automatically hit and deal maximum damage. The target must be within the maximum range of your ranged attack. You can choose to use this ability even after you have made your attack roll, but before you know the result of the roll.
Bleeding Wound
Requires Close Quarters Shooter or Dueling fighting style
You are as vicious as you are precise with your use of your attacks, inflicting excruciating, lingering wounds. Whenever you deal damage to a target with the weapon associated with your fighting style, the target takes additional damage at the end of its next turn equal to half the initial damage (rounded up).
Ultimate Defense
Requires the Defense fighting style
You have become a walking fortress. While you are wearing armor, you gain resistance to piercing, bludgeoning, and slashing damage, even when the damage comes from a magical weapon or source.
Brutal Weapon Fighting
Requires the Great Weapon Fighting fighting style
The thunder in the distance is not from lighting, but the shockwave of your destructive assaults. Whenever you wield a weapon in two hands that has the two-handed or versatile property, you may roll the damage twice and take the best result. If any rolls are a 1 or 2, they are instead considered to be a 3.
Supreme Mariner
Requires the Mariner fighting style
You may hold your breath indefinitely with no ill effect and take half damage from falls. In addition, as long as you are not wearing heavy armor or using a shield your swimming speed and climbing speed equal double your normal speed.
Marvelous Bulwark
Requires the Protection fighting style
You become safety incarnate for those around you. You no longer require a reaction to activate your protection ability. In addition, when a creature you can see successfully attacks a target other than you that is within 5 feet of you, you can use your reaction to halve the damage dealt by the attack. You must be wielding a shield.
Terrible Onslaught
Requires the Tunnel Fighter fighting style
Nobody gets by you unscathed and your weapon flashes and dances in a blur. Opportunity attacks never require you to use your reaction, allowing you to benefit from your Tunnel Fighter ability without entering a defensive stance as a bonus action. In addition, creatures that attempt to move away from you in combat always draw an attack of opportunity, even if they use the disengage action. You can even make opportunity attacks against creatures that have abilities that prevent such attacks, but you make these attacks at disadvantage.
Two-Weapon Master
Requires the Two-Weapon Fighting fighting style
Your grace and speed wielding two weapons is simply unmatched. Whenever you use a bonus action to attack with your offhand weapon, you can make two attacks instead of one.
Some notes:
1) I realize some of these don't have the greatest names. I'm open to suggestions.
2) I couldn't think of one for dueling, so I just paired it with the Close-Quarters Shooter fighting style.
3) In my games, Close-Quarters Shooter has been changed to the following:
Close Quarters Shooter
You are trained in making ranged attacks at close quarters. When making a ranged attack while you are within 5 feet of a hostile creature, you do not have disadvantage on the attack roll. Your ranged attacks deal a +2 damage bonus to targets within 30 feet of you.
4) I operate on the basis that the level 20 ability should be quite powerful as well as unique. Please keep this in mind when providing critiques. I would like to have these abilities balanced with the other classes' level 20 abilities. However, I already feel several of them are fairly weak and I'm working to improve those as well.