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It's a neat idea. The original war of magic used to have escape zones. The problem I had with that old system is that it was too easy to attack, do a tiny bit of damage, and run away. Although guerilla warfare has been around for years it does not always translate into fun. I've had many fights that I've turned around from an seemly unwinnable situation just by thinking tactically. For instance using a pioneer to take a few hits to save the escorting hero. The cautious ability I think is enough as well as the limited escape scrolls. It's a fairly easy thing to mod if you want to add the functionality to the game though. 

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All the tactical battles with an escape tile, but I'm not sure if they still have the functionality. (You are going to have to play with their map editor)

The escape tile still works but only for the Sov and Champs which if I remember correctly was how the E:WoM versions were as well.  However the new retreat function from cautious sov allows you to retreat and keep your troops.  So it is one or the other.