This is why I opened the ideas up to everyone. From experience, I don't fully trust myself to make a plot/setting with sufficient depth on my own. It's much better to take ideas from many people.

I like the idea of one major central empire with minor nations/tribes surrounding it. That does better fill our purposes. It definitely sets a good focus for conquest.

I am aware that the four elements I suggested are clichéd. I am very open to alternative suggestions. In my opinion, nomadic shaman/nature tribe is a good idea, and Storms too (maybe they worship the weather). Although I think Metal is too specific, and Heat just sounds like it will end up being a different name for Fire.

Humans need to be live. They substitute imps, which are necessary for the operation and construction of any Dungeon. This means they can be easily captured/traded from other Keepers, or sabotaged, or liberated by other humans. And it adds a deeper layer of strategy than simply murdering them and absorbing their souls. If you like, an alternative to keeping them content (or oppressed enough so they can't rebel) would be to brainwash them, although that could have the unfortunate side effect of reducing their ability to perform independent tasks. Of course, they still have to be fed.