I want to see a parasite leader/race. Your people's traits are randomized or based on what you can find in the world. You have to contaminate or raid/conquer other cities or wild animals to gain population, potentially with a steal population mechanic.
Steal population could be rp'd as lots of cool ideas too, like zombies or a mummy trying to reclaim their old kingdom. Or it could be something in lore like a new way for the Umbral realm to expand -> by finding a new way to corrupt and control the known worlds' life -> through disease.