I think a great diversion would be adding siege engines for all factions and some would be machines like trebuchet only for Gondor. Isengard would have his strong ballistae, and each faction could have siege towers, battering rams, catapults, or whatever you wish. And it would be nice to better adapt the gates of all factions if they had their own special strength so that it would have to be destroyed with a battering ram, not that any better goblin comes, clicks the right button with his hand on the gate of Gondor and the entire evil army is let into the city. I recommend taking inspiration from various movies, especially from the Lord of the Rings trilogy and The Hobbit as trolls who wear siege machines with goblins or a brick troll. You can also get a lot of inspiration from Battle for Middle Earth 2 Witch King.