I had a thought, about the role of ballista in SEE...
In the Movies, I can't recall ever seeing Isengard uruks using ballista (do they?), but in BotFA the dwarves do. No catapults though. So, what if we use the Isengard ballista for dwarves instead and move the dwarven catapult to Isengard in return? Naturally the operators would be rerigged to proper units.