A completely untested house rule idea for Pathfinder, D&D 3, or other figure chess roleplaying games.
When defending in melee, the defender must make a choice before the attack: either take -2 AC or after the attack has been resolved move five feet away from the attacker, as though taking a 5′ step.
When attacking in melee, the attacker must make a choice before attack: either attack at -2 or after the attack has been resolved move 5′ towards the square the opponent occupied if there is space.
To select movement the defender must have space to move into. The attacker must have space to move if the defender moves or falls. Otherwise, -2.
The rule would allow people to move farther than usual without attacks of opportunity by attacking or defending against incidental attacks. Hence there must be a limit.
I am not happy with the following solution, since it is inelegant. If the character about to move has movement left this turn (a move action, say), then the movement speed of the character is reduced. If they don’t have any (more) movement left, then the speed at which they could move next turn is reduced.
This should be considered and worded more carefully.
The rule only applies to melee attacks. Maybe avoiding ranged attacks and making reflex saves movement to random direction or -2 AC/reflex save?
The rule does not apply to attacks of opportunity.
For a medium-sized creature, moving away from an opponent means any of the three squares that are farthest from the enemy.
There will be unintended conflicts with various class abilities and presumably feats.
One may or may not consider this rule realistic. It would create more dynamic combat situations even with characters that have no special abilities. It would allow pressing through enemy lines or keeping a formation (at a price). Having space behind you would be a boon. Overpowering the enemy with sheer numbers might become somewhat easier.