I was thinking on His Majesty the Worm and how it increases the strategic depth in OSR combat without cluttering the rules the way a lot of tactical RPGs do. As a result, I was trying to think of other simple universal mechanics that encourage more thought in combat, and am thinking about active defense mechanics as a result.
Essentially, all I'm planning to do is make it so that making a saving throw to resist/avoid something takes a reaction. This means that if your reaction has already been spent, you autofail any saving throws that are sent your way.
This means combatants can do things like bait opponents into making an opportunity attack to set up for an undodgeable lightning bolt, or distract an enemy with a grapple so that your mage can try to mind control them.
My main concern is that this may make combat too reliable and not sufficiently frantic, since if you can provoke 2 saving throws you can guarantee that one of them will work (unless the target has extra reactions). It also could feel bad to be on the receiving end of, since players can often be forced to take a loss of some kind (which, with OSR fragility, may mean they're effectively guaranteed to go down or die). It could also potentially create some narrative dissonance, since a powerful orc could still easily get shoved to the ground if there's two halflings instead of one.
Currently I'm considering resolving the issue by giving players 2 reactions per round, and then allowing reactions to be used to defend against attacks as well.
What are some systems that implement active defense mechanics like this? How do they avoid the issues that the mechanic creates? Does the mechanic actually create strategic depth? Any help is appreciated.