So, one of the things that I think are coolest about battles in some of my favorite movies, like the Princess Bride or the Star Wars films, are how when the climactic duel occurs, the fight ranges all over the place. Pathfinder doesn't really emulate this well, which is why I've thought this variant up.
Whenever a character succeeds on a melee attack that targets AC, in addition to dealing damage, they can also push the enemy they hit 5 squares. This represents the foe having to fall back in the face of the character's assault. The character can choose to follow the enemy or not as a free action, taking a 5-foot step that has to end by the foe.
This happens with each attack, so a foe can be pushed back quite far by the furious assault of a well-trained fighter whose full attacks can force an enemy against the wall.
However, if the attack against AC fails, the enemy has successfully deflected the character's attack, and gets to push the attacker back 5 feet. Like the attacking character, they can choose to 5-foot step next to the attacker or not.
This is a rough outline of the concept, so there's probably some balance issues. Still, what are the thoughts on this?
Do you mean "push the enemy they hit 5 feet"? 5 squares is more than the base movement of some player races. (Having Champions flashbacks here.)
This is a lot like the feats Pushing Assault, Repositioning Strike, etc. I think these feats are meant to create the kind of effect you're looking for, and creating a general rule that lets anybody push people around undermines the "specialness" of characters taking these feats. If you want lots of movement, I think it would be better to create your NPCs with feats that create and benefit from movement, and encourage your dueling players to do so as well.
A lot of the movement you see in Princess Bride or Star Wars isn't just forced movement... it's looking for tactical advantage. The "high ground," so to speak. I think this would be a better place to focus on rules changes... if you want people to jump on tables, low walls, etc give a bonus or penalty for a height difference between opponents, for jumping at a foe from above, things like that. Basically, find a way to reward the character for moving around.
(In D&D 4E, they literally did this with monsters... a goblin spearman got a +2 bonus to-hit when it moved at least 20 feet before attacking, things like that.)