I have two suggestions that will enhance the immersion of alliances and heroes in general.
What if heroes could be assigned to allied armies as assistence, so long as they remain in the reinforcement range, they could appear as imbedded and be controlled by the player himself. Alternatively, the said hero's could be accompanied by a number of regiments depending on their level to assist allies.
A regiment of S. of Averlorn for an hand maiden, or two regiments of spearmen for a noble, or a unit of swordmasters of Hoeth for a loremaster. This would only work for allies, so you cannot reinforce your own armies in this manner.
This would allow a faction like High Elves to get involved in Old Word politics, supporting allies without having to send 20 stack armies that are expensive and hard to train. This would also give heroes more versatility.
This idea made sense to me when I remembered the crucial role played by Elven reinforcements in LotR. Without the addition of several hundred elven soldiers, helms deep would have been overrun long before rohirrim arrived.
What do you think of this?