By default, there is an element of retaliation in defending with style, where you gain a boost, but what about using simultaneous combat rules instead? In simultaneous combat, attackers take damage on an unsuccessful attack proportionate to the success of the defender.
John, a player, rolls +5 when trying to attack an enemy.
Said enemy rolls +6 for its defense. It retaliates, dealing 1 stress to John.
What effect does this have on the way the game runs?
It would have to make sense in the narrative, which it might not. A lot of combat behaviours don't have clear retaliations:
As such, it would require a lot of GM intervention to constantly describe how this happens or a lot of exceptions to the rule in general.
If you want to handle it mechanically, it's better to use a Stunt for it. I think there is one in the Fate SRD as an example, but I cannot find it. But I think it went something like
When you Defend with Style in melee combat, you may choose to inflict a 2-stress hit instead of receiving a Boost.
That ties it to the character and rules out most situations where it doesn't make sense, without requiring modifications to the main rules.
I think it would be a great idea to play this way. The problem is: how do you handle something else than duels?
When you only have 2 opponents, that's easy. But when 3+ characters are fighting together, you have to figure a way to cope with initiative, which is easier to deal with when combat is asymmetric.
You'd have to do away with Conflict rules I believe, which work only upon an asymmetric-combat assumption, and adopt an ad-hoc resolution (i.e "He does that, what do you?", "I respond with that", and figure out which skills to roll) everytime a fight happens.
