Philosophically, more tiny reactions is a gameplay problem.
If you have a pile of small impact things you can do off turn, the game bogs down. The narrative impact of each small thing is small, they may add up to only a medium impact; but evaluating a small thing takes the same time as a larger thing.
The multiple attacks on your turn tend to be focused on one foe, so don't have as much impact. People roll them all at once.
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Rather than give people a pile of reactions, maybe make reactions more beefy. And maybe give people a chance to not lose your reaction, so you can't say "distract him, then flood past".
Threaten
When you make a melee attack on a foe on your turn, you threaten them. If you fail a saving throw or are hit before the start of your next turn, your threat goes away. If you start your turn threatening a foe, your attacks on that creature are at advantage, and the first hit is automatically a critical hit.
Opportunity Attack
When a creature tries to get past you by leaving your reach or moving through your space, you can expend a reaction to make a melee weapon attack. If the attack hits, you Threaten the creature. If your attack roll is even, you regain your reaction.
This does 2 things.
1. Threaten is an anti-focus-fire feature. Creatures you don't attack get threat, which they turn into punishment. So you are encouraged to attack creatures who are hitting you instead of focusing on bringing one foe down.
Similarly, monsters are encouraged to spread their attacks around instead of focusing on dropping one player.
Mooks can offer cover for the big bad; a rogue threatening the big bad can have that threat cleared by a mook. But you have to hit or make them fail a save to block the threat.
2. Opportunity attack makes ignoring the front line worse.
First, you have a 50% chance to get a 2nd OA, etc. Second, it is a way to apply Threat off turn. Off-turn Threat is much stronger; there is less time for someone to clear it.
So if you run past the front line, you get to hit. Then if they keep going, you can follow and clean up with auto-advantage auto-crit, unless you are kept busy by their allies.
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I am worried it is too expensive however.
If you have a pile of small impact things you can do off turn, the game bogs down. The narrative impact of each small thing is small, they may add up to only a medium impact; but evaluating a small thing takes the same time as a larger thing.
The multiple attacks on your turn tend to be focused on one foe, so don't have as much impact. People roll them all at once.
---
Rather than give people a pile of reactions, maybe make reactions more beefy. And maybe give people a chance to not lose your reaction, so you can't say "distract him, then flood past".
Threaten
When you make a melee attack on a foe on your turn, you threaten them. If you fail a saving throw or are hit before the start of your next turn, your threat goes away. If you start your turn threatening a foe, your attacks on that creature are at advantage, and the first hit is automatically a critical hit.
Opportunity Attack
When a creature tries to get past you by leaving your reach or moving through your space, you can expend a reaction to make a melee weapon attack. If the attack hits, you Threaten the creature. If your attack roll is even, you regain your reaction.
This does 2 things.
1. Threaten is an anti-focus-fire feature. Creatures you don't attack get threat, which they turn into punishment. So you are encouraged to attack creatures who are hitting you instead of focusing on bringing one foe down.
Similarly, monsters are encouraged to spread their attacks around instead of focusing on dropping one player.
Mooks can offer cover for the big bad; a rogue threatening the big bad can have that threat cleared by a mook. But you have to hit or make them fail a save to block the threat.
2. Opportunity attack makes ignoring the front line worse.
First, you have a 50% chance to get a 2nd OA, etc. Second, it is a way to apply Threat off turn. Off-turn Threat is much stronger; there is less time for someone to clear it.
So if you run past the front line, you get to hit. Then if they keep going, you can follow and clean up with auto-advantage auto-crit, unless you are kept busy by their allies.
---
I am worried it is too expensive however.