Monks of the Way of the Sentinel follow a tradition that values the strength of life. Through time they have been called many names, and they serve as guardians and warriors. Monasteries that teaches this tradition can follow many different causes. Some are loyal to a kingdom and other have found their own cause. Usually they align towards good, but some monks stray from the path. They are usually known as fallen sentinels.

Sentinel Gaze

3rd-level Way of the Sentinel feature

You gain proficiency in Investigation and Perception skills, and whenever you make a Investigation and Perception, or a Initiative roll you can roll your Martial Art's die and add the number rolled to the ability check.

In addition, your gaze becomes unwavering. If you spend 1 ki point or more as part of your action on your turn, you gain blindsight equal to fives times your proficiency bonus until the end of your next turn.

Sentinel Strike

3rd-level Way of the Sentinel featured

Whenever you hit a creature with one of the attacks granted by your Flurry of Blows, you can mark one creature until the end of your next turn. While marked, the target can’t become hidden, and if it’s invisible, it gains no benefit for the duration. 

In addition, when you make a opportunity attack, you can expend 1 ki point when you hit with the attack to grant this benefit. 

Variant Rule: Improved Deflect Missiles-Deflect Strike

6th-level Way of the Sentinel feature

Your Deflect Missiles class feature has been improved by taking this subclass. You can now perform the feature on melee attacks against you, following the same restrictions and requirements. If you reduce the damage to 0, If you reduce the damage to 0, you deflect the attack. If you stop the melee attack in this way, you can spend 1 ki point to make an melee attack against the attacking creature as part of the same reaction. 

This is a class feature changed that most players of mine that had the 3.5 background requested to maintain the monk identity. It fit's pretty well on any monk-subclass around this level. But this is considered to be a bonus to the class and not a core sub-class feature and will need further DM approval. 

Sentinel Stance

6th-level Way of the Sentinel feature

When you use your Patient Defense, you can awaken your ki to become like adamantine. Any critical hit against you becomes a normal hit and when you are subjected to an effect that allows you to make a saving throw to take only half damage, you instead take no damage if you succeed on the saving throw, and only half damage if you fail.

You can use this feature a number of times equal to your proficiency bonus, and you regain all expended uses when you finish a long rest. While you have no uses available, you can spend 1 additional ki point when you activate Patient Defense to use this feature again.

Sentinel Presence

11th-level Way of the Sentinel feature

Your training has formed your body and ki to create a overwhelming pressure surrounding from you, hindering your foes as they move within it. As an action, you can creature an aura of oppressive force that radiates 30 feet from you in all directions for 1 minute. For the duration, It hedges out vapor, gas, and fog that can be dispersed by strong wind and ranged attacks from hostile creatures have disadvantage against you. When a hostile creature enters the radius for the first time on a turn or starts its turn there, it must make a Strength saving throw or take a -2 penalty to AC and have it's speed halved until the end of your next turn. 

Once you use this action, you can’t use it again until you finish a long rest, unless you expend 4 ki points to use it again.

Overseer

17th-level Way of the Sentinel feature

As a bonus action, you can spend 5 ki points to awaken the dormant ki within you to empower yourself for 10 minutes. This awakening ends early if you are incapacitated or die. For the duration you gain the following benefits:

• You gain truesight and can see into the Ethereal Plane, all out to a range of 60 feet.
• You can apply your Sentinel Strike to any target you hit with your unarmed strikes without spending the ki point, not just once with your Flurry of Blows.
• When you attack a creature with your Sentinel Strike mark, you have advantage on the attack roll.