Well, not sure if this is good idea... but maybby worth to have small thought on it.
Siege engines now are useless without actual siege, and can be used only against walls. But actually siege engines were used on both side of siege and on the open field aswell(catapults etc.).
So, how about to add totally new troop type? Ranged siege-engines or catapults. At battle they will fire at ranges 3-6, mayby every second round, doing pretty massive damage if they hit. And ofcourse those would be lost if battle is lost and unit retreated.
This could be hard to do... but just thought it would be cool to have catapult!