So, the Miss Step idea evolved from the mention of the Drunken Master subclass getting a free disengage when you use Flurry of Blows (plus an extra 10' speed for that turn). This also seems related to Step of the Wind.
Looking at Step of the Wind and features tied to it:
- Step of the Wind gives you a Disengage+Dash combo as a bonus action for 1 DP. The rogue's Cunning Action can do either Disengage or Dash as a bonus action for no cost.
- Warrior of the Hand gets Step of the Wind for no cost starting at level 11.
- Warrior of Shadow can teleport through shadow with a bonus action at level 6, and can spend 1 DP at level 11 to not require the shadow. It also gets a free attack (with advantage) with the level 11 version. (This is not Step of the Wind, but has similar purpose.)
- Warrior of the Elements gets a fly (and swim) speed when using Step of the Wind, starting at level 11. At level 17, it also grants an extra 20' of movement, and does damage to anyone you step next to.
And looking at the action economy of the bonus action, your competing options are:
- Make an unarmed strike for free
- Make two unarmed strikes for 1 DP (Flurry of Blows)
- Disengage+Dash for 1 DP (Step of the Wind)
- Teleport + 1 unarmed strike for 1 DP (Improved Shadow Step, level 11)
There are other options, but less related to the current topic, so I'm not going to include them here.
So our baseline bonus action is an unarmed strike. Spending 1 DP on Flurry of Blows to increase that to two unarmed strikes seems entirely reasonable.
Cunning Action shows that it's entirely reasonable to have Disengage or Dash as a bonus action for no cost. Step of the Wind has a cost because it combines the two into a single bonus action.
The free Shadow Step (level 6) is essentially a Dash+Disengage move, since it's a 60' teleport. You only pay DP on the higher level because it adds damage (similar to Flurry of Blows) and avoids the shadow limitation.
Still, as a baseline ability, I don't think I would disagree with Step of the Wind having a cost.
Now, because of how monk is designed for damage, losing your bonus action attack is pretty significant. That's 1/3 to 1/2 of your damage output. A rogue might give up his bonus action for Cunning Action, but his primary damage is Sneak Attack, so very little is lost unless he misses entirely. Other jobs similarly put the bulk of their damage in the main attack action.
Because so many things revolve around a monk's bonus action, including almost half of the class's potential damage, using it for small gains (such as a 5' disengage) feels like a poor application. At the same time, it needs more viable actions that don't require DP.
Is it adequate to have a full Disengage as a no-cost bonus action, similar to Cunning Action? Honestly, it doesn't feel right. Aside from being more of a rogue thing, it doesn't feel like it fits the thematics of the monk. A 5' disengage is almost entirely worse than that.
However the example given of kicking off the enemy's shield in order to get space does seem more in keeping. However, while it was framed based on the pun on Miss Step to only be granted when you miss, I think it would fit better if it worked when you hit.
So then the question is, is a bonus action unarmed strike + Disengage a valid no-cost option? And while I don't think it's worth a full DP, I also don't think I could agree with it being free.
But... What about a different cost? 1 Unarmed Strike is considered the value of 1 DP (see: Flurry of Blows), so maybe half a DP? In other words, half an Unarmed Strike. And we can put the tweak on the base martial arts ability.
Bonus Unarmed Strike: When you use the Attack action with an Unarmed Strike or Simple Weapon on your turn, you can make one Unarmed Strike as a Bonus Action on the same turn. If you hit with this Unarmed Strike, you can choose to do half damage and get the benefit of the Disengage action.
If you missed with the strike, you're out of position, and would have a harder time moving out of the way. But if you hit, you can use the recoil from the strike to push you out of the way before the enemy can strike back. By using part of the damage to push yourself away (thus doing half damage), you can still get the visual effect of backflipping off the enemy's shield.
I'm not sure about the difference between a full Disengage, and just 5' of movement that doesn't provoke Opportunity Attacks. Can polish that out later.
Patient Defense
As a reaction, you impose disadvantages on an attack against you.
1DP: disadvantages on all attacks until the start of your turn.
And I do like this. Effectively a built-in Blade Ward for the class. Have the same melee limit to not clash with Deflect Missiles.
Defective Missile
As a reaction, reduce the damage of a ranged attack
1DP: it does not take your reaction.
And using the DP to make the deflection not take a reaction I think is a really nice option.
Defensively you'd be better off using Patient Defense's Dodge for 1 DP, but if you didn't plan for that, you could still burn a bunch of DP to deflect several missiles and 1 melee attack from the passive Patient Defense benefit.