Well, I would create individual siege weapons for each race (for example: elves - as they are not fans of machines and all sorts of dark technology (see race description), they could make an ENT instead of a catapult, which would throw stones (like a catapult against walls), and instead of a ballista, I don't know what else could be invented, etc. By the way, like in Warcraft, you could let the Necropolis have a corpse cart as a catapult!
Added 1 minute ago
100% cool idea! The corpse cart would throw corpses and lower morale!:D