I have been pondering this for a while. It is an interesting question and I think it is going to depend on what you want to achieve. More realism in combat? More unique combat system? Options for players? Something to make players happy?
Based on my experience from teaching martial arts, there are reactions (block, parry, redirect, dodge, evade, interrupting strike, jamming, etc.) with counter being just one type of reaction. Action is faster than reaction and reactions can be purposeful and deliberate (meaning I am consciously planning on reacting in a particular way for a particular reason) or natural and instinctive (meaning based on stimulus and fast than conscious thought).
Regardless, your ability to react is based on many things and, if possible, I would try to link the reaction mechanic to as much as makes sense and is doable. For instance, reactions are influenced by your natural dexterity/agility, your speed, your skill, your experience, and even your intelligence. In my experience it is much easier to fight a person who is both physically and mentally slower than someone who is smart, cunning, and quick. And luck can also play a factor.
Because of the skill part, I like games that incorporate skill systems that increase with use. Realistically, a person doesn't get significantly faster (speed) or more agile over the course of your adult life. At least not in a large scale like 0-100. If you have a 50 speed or agility, you aren't really going to raise to 100. Skills on the other hand can increase dramatically over time.
Don't get me wrong, you can get more agile and faster at a skill. I have seen people start off clumsy and slow when they are first learning martial arts and years later they might be graceful and quick. But that change in performance is as much linked to experience/skill as it is improving a natural attribute.
How you limit reactions (1/round, 1/turn, 1/encounter) is a design choice, but I would consider these things.
A true counter is mostly automatic and is based on a collection of things and immediate stimuli. Generally speaking, it isn't a conscious choice, it is a natural reaction.
The list of other reactions can be both deliberate (planned) or instinctive. I can plan to dodge/evade or parry/redirect/block because I want to protect myself, watch for a better opportunity to attack, look for a weakness or vulnerability, to conserve my stamina, etc.
Or, those reactions can be more instinctive based on my skill, experience, intelligence, etc. and therefore more automatic.
If possible, I would try to find a way to incorporate all three possibilities: instinctive counters based on specific set of circumstances/limitations, planned reactions to gain some sort of benefit in combat, and instinctive reactions based on character skill, experience, etc.
Similarly, Feint is an interesting option. In combat, I use Feint to try to get my opponent off guard and lower their ability to counter/react to my next attack.
Now, that might be too deep for your combat system, too difficult to implement, or potentially too boring and frustrating for some players. So maybe these types of things wouldn't be popular ideas.
I think some games try to incorporate all of this reality into a more simple system. Your attack misses (because of any number of these unnoticed things) or you evade an attack (because of any number of these unnoticed things). Unfortunately, I think many players don't appreciate that a more simple combat system may (or may not) have planned for that and they skipped all of this detail to make the combat system for simple and fun.
I should also mention that this is more for turn-based combat. In real-time combat, the player can look for telegraphing/pre-assault cues in the AI and try to react accordingly as they learn the AI and depending on what type of reactions are allowed.
I am a huge fan of the D&D concept of consequences from a NAT 1 or NAT 20 roll. Some players would say that it is impossible for my Level Godlike warrior to drop their sword. But a NAT 1 system provides for the culmination of the worst luck that can happen while the NAT 20 provides the culmination of the best luck possible. It isn't that the warrior just dropped their sword. Something else significant happened. It doesn't need to be explained (although you could try to incorporate some reasons in flavor type combat text), it can just happen.
Anyway, this post is too long, but I hope I offered some food for thought and that this is somewhat helpful.
I will watch this thread because I am very interested in this question. Thanks!
