[Sawrock]

Here's an idea: when you flee in Serious PvP, you leave your party behind in the fight. In addition, in serious PvP when an enemy party has more player characters than your party does and you try to flee, you get a penalty to flee, as follows:
Amount of PCs are Equal: Normal Flee Chance
+1 PCs on Your Side, Comparatively: 1.25 Flee Chance Multiplier
+2 PCs on Your Side, Comparatively: 1.5 Flee Chance Multiplier
+3 PCs on Your Side, Comparatively: 1.75 Flee Chance Multiplier
-1 PCs on Your Side, Comparatively: 0.75 Flee Chance Multiplier
-2 PCs on Your Side, Comparatively: 0.5 Flee Chance Multiplier
-3 PCs on Your Side, Comparatively: 0.25 Flee Chance Multiplier

In addition, fleeing could have a max chance of 75%, thus making it a gamble option, and if you flee and fail, then Flee gets a cooldown of three turns or something.

Edit: Here's an explanation of why I suggest this: it allows for sides that have a numerical advantage to thin their numbers for IC reasons (Oh, the guard is here, they want to take over for you, citizen-who-has-been-attacked!) and if you DO decide to flee, you'd be abandoning your allies, which can have IC repercussions (no one wants to be known as the traitorous coward).