I find that the battles are totally isolated to just you, battling your enemies, without any help (ever) from any of your allies (even though they quite close by).

So maybe changing the way players and AI take their turns may be able to make battles much bigger. In other words more armies will be able to join a particular battle at one time.

So the idea is to have player and AI make their turn and movement like normal BUT battles will only resolve after the ROUND has ended, instead of resolving them during that turn itself.

This will mean that multiple armies (whether the player or his allies/even enemies) Can join a particular battle, which will resolve only at then end of the round. Thus this can cause huge battles on a short notice, instead of just attack an enemy alone every round even though your allies may be in range to support (marching range not beside)