I'm starting to think that Zayth needs an invasion. Lets see how long they fight each other when they find a common enemy, but I'm unsure who the invaders should be Orks(maybe), Rak'ghol (this might be intresting but they need to be fleshed out more) or Chaos marines with cultist in grand scheme turn this into Khornite playground (not really sure about this). What else could invade world with macrocannon toting mobile cities
Still I always get mental image traction cities from mortal engines series of books by Philip Reeve even they are lot steampunk/apocalypse. Unless you've read the books feel free to peruse wikia about it
http://mortalengines.wikia.com/wiki/Mortal_Engines_Wiki
to get idea about some concepts in it. I think this could be a good flavour addon to Zayth
./atomiclich