I'm pretty sure the counterattack mechanic already exists, the swordsman in my party often tries to counterattack its attackers...
Also, there is a Sorcerer 3rd lvl spell called Counterattack. However i don't know if it allows you to counterattack if the opponent fails.
That would make for a whole lot of rolling to match every attack. Rite Publishing released some new combat maneuvers in their Secrets of Adventuring series that are worth checking out. Also, there are a few books that spice up regular combat such as the wu xia styled Dragon Tiger Ox supplement, Martial Arts Guidebook or Dungeon Grappling.
I added something like:
* If you still have a move action you may choose to roll in response to any attack roll you may take a Reflex save with a DC equal to the rolled attack value of the opponent. If you do, you ignore that attack on a successful save, otherwise you take the damage as normal and lose that move action.
Either way, after this has been resolved, move 5ft in any direction exactly as if you had taken 5ft free step, except that you are allowed to do this even if you have already used your 5ft free step or moved.
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This makes dex classes more powerful and offers a reason to save your move actions rather than just full attack every round. I am going to be testing it in coming weeks, but my players rolled a couple of dex classes to try it out so it should be exciting!
Note; this is in addition to giving most of the "improved" combat maneuvers for free to PCs to allow them more realistic options in combat (in terms of "realistic success without taking unnecessary AoO's")
We tried the roll instead of take 10 for AC for a while because one player wanted it and it sucked. It made playing high AC chars uselesss because you could not trust your armor and it lengthend the time of combats significantly.
Isn't this just the parry an riposte class feature for swashbuckler?
(Not sure who it's in response to)
Only in that it's a D20 optional defense mechanic.
* SB uses your AoO, mine uses Move or Standard actions left over in your turn (which means you miss out on a full attack to do it.)
* Mine leaves your AoO in place
* Mine uses a Reflex Save so is a defensive mechanic that benefits Dex players. Dex classes happen to be classes that are generally less powerful
* P and R gives you an attack. Rolling doesn't
* Rolling moves you 5ft, which is a giant defensive buff as well. Why? Well because if you do it on the first attack of an enemies' full attack, it means they will likely waste attack actions if no one else is around.
This actually means that an SB in my game who did a single attack on their turn (and free/swift actions) could still do a Parry or Roll and then be able to do the one they didn't use on the next attack.
The intended benefit at my table is to purposely cause more choices in melee. By giving free buffs to the combat maneuvers (most of them I hand out Improved for free) and by giving the rolling option, I want full martial characters to feel like they aren't punished for things; but rather have options.
I similarly buff opponents with Grab, Trip, Awesome Blow, or other things (I'm careful and choosy about it, but still) in order to make Full Attacks have a detriment to it. Don't want to be grabbed? You should keep your roll around!