Pugilist

While monks dedicate their lives to diligent study of the martial arts, the pugilist takes a more practical approach to unarmed combat.  Rather than monasteries and academies, the pugilist's art has been developed, tested, and proven in arenas, fighting pits, and seedy barrooms the world over.  While the style is more brutal and less disciplined than that practiced by most monks, it is no less effective in combat.

Unarmed Fighting

When you choose this archetype at 3rd level, you gain the Unarmed Fighting Style.  If you already have it, you may instead choose another fighting style from those available to fighters at 1st level.

Furthermore, your Unarmed Fighting Style improves as you gain levels.  At 11th level, your unarmed strike damage increases to 1d8, or 1d10 if not using any other weapons or a shield.  At 15th level, your unarmed strike damage increases to 1d10, or 1d12 if not using any other weapons or a shield.

Haymaker

Also at 3rd level, you know how to put your weight into an attack to make a telegraphed--but powerful--blow.  When you make an attack with an unarmed strike on your turn, if you do not already have disadvantage on the attack roll, you may choose to make the attack with disadvantage; if the attack hits, it deals additional damage equal to your STR modifier (minimum +1).

Fancy Footwork

At 7th level, you know how to stay light on your feet without letting your guard down.  Whenever you take the Dodge action, you may Dash or Disengage as a bonus action.

Clinch

At 11th level, you know how to get in close with an opponent in order to prevent them from withdrawing from a fight.  Whenever a creature provokes an opportunity attack from you, you may instead use your reaction to make a grapple attempt.  You may use your reaction this way even if the creature is using the disengage action.

Guard Up

At 15th level, your experience with fighting unarmed has honed your defensive instincts.  Whenever you take the Dodge action or make at least one attack with an unarmed strike during your turn, you gain +1 AC until the start of your next turn.   You do not gain this bonus if you are using a shield.

Knockout Blow

At 18th level, your unarmed blows are powerful enough to knock an opponent down for the count.  Whenever a creature of Large size or smaller takes damage from your Haymaker ability, they must make a CON saving throw against a DC equal to (8 + your proficiency bonus + your STR modifier.)  If the saving throw fails, the creature falls unconscious for 1 minute.  They may make the saving throw again at the end of each of their turns, whenever they take damage, or if another creature uses an action to attempt to shake or slap them awake; the creature regains consciousness if they succeed on a saving throw, but remains incapacitated until the start of their next turn.

Once a creature regains consciousness from your Knockout Blow, they know not to underestimate your strength and are immune to your Knockout Blow for 24 hours.