That's pretty cool. I may adopt that for my home game.
I was toying with the idea of changing the way flanking worked for my game. Instead of making it a situation dependant on the attacker, I thought of making flanked a condition, imposed on the defender. It's a subtle difference, but one that certainly would have a consequence in the game. The flanked conditon would still rely upon two people attacking from opposite sides, but once that was achieved, all attackers would gain the benefit.
My reasoning for the change is this:
If a defender has to worry about attacks coming in from multiple sides, then he/she/it should be distracted enough to affect all incoming attacks, not just those two individuals that happen to be at exactly 180 degrees apart.
I think all PCs would benefit, but ranged combatants and rogues would see the biggest benefit. Of course, this change could work against the party, but in my experience, it would help the PCs more often than hurt them.
I was wondering what other people thought about this. Is there some "butterfly effect" I'm not considering? Does the rule change make sense?
My group has used this very house rule for more than a year now.
If a defender is flanked by attackers, that defender is treated as flanked (we call it "harried") for all other attackers.
And you absolutely nailed it, ranged attackers and rogues (and especially ranged attacking rogues) love it.
As a DM I have yet to observe any situation that made me think "OMG guys this is too overpowered".
And if you find problems with it in your game...just tweak it or go back to RAW. It's just a house rule.