Just a suggestion if you want to add more vanguard leaders, what about make enemy general can defect and join Central vanguard leader? You need to add in battle mission to make him/her surrender.
Here is the example in battle mission, you can use this if you want:
Capture or burn enemy supply depot and make target and his/her troops starving and wait until he/she surrender. Player must hold back, not kill him. If player kill him before they burn enemy supply depot they lose chance to make him vanguard leader. After player success make him surrender, they can launch full scale attack against remaining enemies to win the battle.
In other words, this mission different from Lind story, defected general is not main leader, and make him surrender is not main objective, whatever player choose to make him surrender or defeat him, they can win the battle if they defeat all enemies.
I think add in battle mission will make the battle more interested to play.