This fighter might be a former duelist, trained in the art of reading their opponent's intentions and capitalizing on even the smallest opening. Perhaps they belong to an order of martial artists who emphasize anticipation and redirection over raw power. Their fighting style is characterized by fluid movements and an almost preternatural ability to react to sudden changes in combat. Their opportunity attacks are swift and unexpected, turning an opponent's aggression against them.

Incoming weakness

You can now do a opportunity attack on a enemy that gets in your range 

Multiple weaknesses

You can now do as many opportunity attacks as you want without using your reaction 

Stronger opportunity

Your opportunity attacks now do 3d4 extra damage and the target goes to half speed for 3 turns 

Further opportunitys

You can now do opportunity attacks from 10ft away 

Tiredness opportunity

You can forgo all attacks next turn when doing a opportunity attack if you do that attack does 3d10 more damage and the target can’t move this round and has a minus 1 to every roll