Well this is one side affect that comes with no zoning. You can't have mobs chase you forever. Mobs chase you back to a city entrance??
I to miss the times when your out and about and one of your party members catches aggro and they have to run back to party for help...or die. This running in the opposite direction for 10 seconds is very easy but what do you suggest with no zoning? I think its either players have zones themselves or the mobs have zones (in which they disengage once reached).

They could make it harder to run from them. Pick up the speed a bit and not give up if the player is still within a certain range. Need more diverse landscape that make it a challenge to run. To open and just run in line and mob is gone. Be better if we had to run around bushes, forests, had edges that got in our way...this will cause players to switch directions which may give mobs a chance to catch up and engage. Gives some mobs the ability to bind you to a spot for a certain amount of time. Mostly used on fleeing players. anything to spice things up a bit.