While i realy like the current battle zone mechanics, there is one issue that is very signifficant in multiplayer, the zones are to small in battles with 50+ units. so that u can't deploy all rienforements. 
This is especialy noticable as the defender, since the attacker can pick how the battle layout looks, and therefore make sure the defender can't deploy all his units on the battlefield.

In the picture below, a example from early classical, where blue will lose the battle, since he can't deploy his ranged stacks and can't bring in any new stacks from behind(there is 20+ ranged units just behind the battlefield) , and does not have a single tile to move his cavalery around because the initial battlefield generation gave the mayority of the battlefield space to the attacker.

Suggestion: In battles with 50+ units, increase the battlefield size zu industrial era scaling

The same issue happens in gunpower age, when u have battles with 200+ units, and the battle is won or lost, by who ever get's lucky enough to be generated space that permits the deployment of more motars than the enemy is permitted to deploy