Forgive me for the provocative title.

There is one thing I would like to see for the monk, which I haven't seen mentioned in all the discussion. It have nothing to do with power level, and everything to do with the class fantasy. That thing is the concept of turning the enemy's power against them. It's a well-established idea both within real-life martial arts and in fiction, but it's completely lacking in the game.

I do think it's difficult thing to represent with D&D's turn-based combat system. You could grant the monk a bunch of reaction options, but they would still only get one reaction per round. Maybe some mechanic that depends on things that happened in the last round, but that might be cumbersome to keep track of.

How would you add this feel to the class mechanics? 

I think it has already been suggested before: making deflect missiles also work at melee. As it is now the feature is mostly used for defense only, seldom will the chance to return the damage ever appear since it has 4 different costs/conditions

1. Only works for ranged attacks (not saving throws)
2. Damage from the attack must be reduced to 0 to even attempt to retaliate
3. Is a reaction so it only works against a single attack (I don't have a problem with this one, but many people will always complain about action economy).
4. Costs a Ki/Discipline Point.

The base feature should work with melee attacks as well and remove the Ki/DP, having a cost on top of a not guaranteed condition is annoying.

Another option is like you said, making the monk the "reaction class" with "monk reactions" and having them scale and unlock with monk levels, and maybe a subclass that grants special reactions specifically for "monk reactions". Cavallier is like that, a Monk subclass could be as well.

Drunken master has a feature to redirect an attack to another target. 

Redirect Attack. When a creature misses you with a melee attack roll, you can spend 1 ki point as a reaction to cause that attack to hit one creature of your choice, other than the attacker, that you can see within 5 feet of you.

Or Riposte.from Battlemaster is another similar option

Riposte. When a creature misses you with a melee attack, you can use your reaction and expend one superiority die to make a melee weapon attack against the creature. If you hit, you add the superiority die to the attack’s damage roll.

Neither of those have quite the right feel, IMO, though the drunken master one is close. I would expect something like 

When a creature you can see moves into your range and makes a melee attack against you, you can use your reaction and spend one ki point to trip the attacker. The creature makes a dexterity saving throw against it's own attack roll. On a failed save, the attack misses and the creature is knocked prone. On a successful save, the attack is resolved as usual.

 The point is the inversion - the higher the attack roll, the more likely you to succeed. It's kinda awkward mechanically, and also I'm not sure where it is in terms of power level, so I wouldn't add it as-is, but this could be the basis for what I'm looking for. 

Why even make these things cost ki? Maybe just make more options for using the bonus action and reaction a core part of martial arts. The real cost is the use of action economy. 

Then let the ki points fuel things that kind of go above and beyond. 

How about 

You can redirect the force of an enemies attack against them. When a creature hits you with a melee attack you can use your reaction to halve the damage done to you and redirect the remaining damage back onto the attacker. The creature takes the same amount of damage

Not the best wording, but you get the idea. 

I like that, and that's when spending a ki point for a full damage redirection would fit.

Damage that's what its missing the most.

followed by scaling ac and everything not having to cost ki.

IMO, the thing that would be really useful and flavorful for Monks is if they were the only class to get more than 1 Reaction. For instance, at level 7, get to make 2 Reactions. At level 13, get to make 3 Reactions. This would let you Deflect Missiles multiple times or combine Deflect Missiles with an opportunity attack on the same round. 

Quote from LotemNS >>

Forgive me for the provocative title.

There is one thing I would like to see for the monk, which I haven't seen mentioned in all the discussion. It have nothing to do with power level, and everything to do with the class fantasy. That thing is the concept of turning the enemy's power against them. It's a well-established idea both within real-life martial arts and in fiction, but it's completely lacking in the game.

I do think it's difficult thing to represent with D&D's turn-based combat system. You could grant the monk a bunch of reaction options, but they would still only get one reaction per round. Maybe some mechanic that depends on things that happened in the last round, but that might be cumbersome to keep track of.

How would you add this feel to the class mechanics? 

 The developers can modify reactions and what not as much as they want.  Look at UA8 they allow you to spend DP points to get extra attacks, or to disengage and dodge.  They could have easily done the same for Deflection.  I know this doesn't directly address your points, but I think this version of the Monk has promise.  And we might as well get used to it because they may not make another class change until 2034.