(Credit to Destereza for the original Way of the 'Thunderous fist')
You, as a Monk under the tradition of the Thunderous Fist, have learned how to vibrate the energy inside you and shatter all barriers by shoving your body forth through the limits that it desperately holds onto. Your fists deafen even the greatest foes, and the sound you sling at others can pierce the hearts of the most heavily armored knights, and shatter the bones of the mightiest dragons.

Even with this strength, you also learned to move throughout the world like a crack of thunder, your might showing in every part of your body. Each and every step you take feels like a sudden weight upon the world. As you move with speed befitting of the very lighting that is followed by that memorable Bellow of Thunder. Striking hard, fast, and relentless like the storm you take into yourself, for good or ill, you shall make quakes that shake the very world on your path to your own ends. 

Regardless of where it may take you, use the power of the mighty earth-shaking thunder, and use it well.

Might of Thunder

With the Way of the Thunderous Fist training within your mind, you have learned how to incorporate indomitable might into each of your strikes. Starting at 3rd level, you begin this path.

Whenever you make a melee strike against a target when using a monk weapon or an unarmed strike, you can choose for the attack to deal additional Thunder damage equal to your Martial Arts die. You can only use this feature once per turn. 

 When you hit with a monk weapon, or an unarmed strike, and have expended one point of Ki on that turn (Such as patient defense or Fury of blows), you may attempt to force the target backwards. They must make a strength saving throw, and are pushed back by 15ft on a failed save, or 5ft on a successful save. Creatures of Huge or larger have advantage on this saving throw.

Booming Steps

Starting at 6th level after taking this subclass, you have learned how to sense the feeling of sound itself around you, and step with it in tow. You gain two new Ki based features.

Stomp of Thunder: Using four Ki points, you can cast Thunder Step as a third level spell as a bonus action. You can use an additional Ki point to increase the level of this spell by one per additional Ki point used, but only use an amount of Ki points equal to your Monk level, rounded down. This does not apply to the base four Ki points used.

Thunderous Sense: You gain the ability to sense beings that create sound. Using your bonus action, and one KI point, you may gain Tremor sense expanding out to 60 feet from you. This tremor sense lasts for one minute but can be ended at any time as a Free Action. However, to use this feature you must first close your eyes entirely, and become blind. You cannot use this feature if you are deafened, incapacitated, or unconscious. Additionally, when using this feature, you gain disadvantage on Dexterity (Stealth) checks if you move whilst this ability is active, as you send out loud resonating waves of sound from each step or touch of surfaces around you.

One with the Booming Thunder

 

At 11th level, you have begun to become one with the thunderous might you seek to harness. Your Ki points have become intertwined with this power, as your life force has begun changing to embody this newfound strength. Your body has begun to silently vibrate with the energy lurking beneath your skin, and can be felt if someone is to ever touch you.

Whenever you use a technique that requires you to spend a Ki point, you may add additional effects to it.

Flurry of Blows: If you perform this technique after gaining this feature, your strikes can be made to deal thunder damage instead of what they would normally do. Additionally, if you are to hit with both strikes from this feature, the target is deafened until their next turn.

Patient Defense: If you perform this technique after gaining this feature, you gain an additional effect. If you take the dodge action as a bonus action using this feature, you are able to immediately make an unarmed melee attack against a creature that attempts to attack you in your range, and misses, as a reaction.

Step of the Wind: You use this feature to fire a blast of thunderous force between yourself and the enemy. After gaining this feature, and using the step of the wind ability, you gain the disengage ability for free. When the ability is used, you impose disadvantage to the next opportunity attacks of all creatures within 10 feet until their next turn. additionally, you gain a fly speed equal to your unarmored movement bonus (Ex: 20 feet at 11th level), provided whatever direction you move in, is a straight line. The blast ability does not require disengaging to be used and can be used when only dashing. However, the line and distance restriction still apply.

Stunning Strike: When you use this feature, at 11th level, Stunning Strike gains an additional effect. When the target fails the saving throw, one additional foe within 5ft of the target must make a con save. On a fail, they also become stunned as the shockwave travels through their body. 

Stomp of Thunder: This feature does not gain any additional benefits.

Thunderous Sense: You do not have to close your eyes to use this feature anymore.

Roar of Gods

You've gained mastery over the thundering might permeating your body and soul. Constantly active, anyone can feel, or in some cases see, the minute vibrations of energy that runs under the surface, and the sparkling lightning glistening in your eyes. You are a voice of the heavens personified. You- Are the storm.

 

At 17th level, you gain the following features and abilities:
- You are Immune to Thunder Damage and Resistant to Lighting Damage.

-Spend a reaction to add +3 to your AC until the start of your next turn. (With a boom of deflecting force heard up to 100 feet away)

- You can now reliably keep yourself airborne with your bursts of thunder. You gain a Flight Speed equal to your walk speed.

-Lastly, you can charge forward in a burst of a powerful force, breaking the sound barrier on your way to deal a devastating supersonic blow. As an action, you may expend 3-10 ki points to move up to your movement speed, take no opportunity attacks, and attack an enemy within range. The target must make a constitution saving throw or take 3d10 points of thunder damage on a fail, or half as much on a success. Adding 1d10 extra points of thunder damage for each Ki point expended above 3, to a maximum of 10d10.
Additionally, on a failed save, the target is pushed back 20 feet, unless an object is in the way, stopping the movement with collision instead.