I like this idea. Would it be more fantasy or sci fi though? I've never really enjoyed dice rolls either (plus, the other guy might be making it up xD)

I have a suggestion for the economy, maybe it could be similar to Black and Whites power system or Overlord 2's system? Like, if you attract cultists they generate a steady supply of Life Force continually (As you are siphoning away parts of their soul) you can have your army go on a rampage in the mortal world and get a larger, one time amount from harvesting pesky mortal souls. We'd have another, rarer resource that could only be found in the mortal plain, which would be required to combine with the other resources to make more powerful servants and more advanced structures (so its basically got properties that make it a catalyst for evil energies) And the final resource would be simply be Energy, which would be the resource required in order to force your will onto the world, so it would be the one used to construct structures and to launch manifestations of your dark power at your opponents.
Life force comes from slain mortals or from your cultists
The second resource comes from mining it out in the human world (or raiding the mortals who are already in possession of some)
The final resource comes slowly over time, and the amount you get can be increased by spawning a specific structure
As you can see, I got hooked on the idea of it as a quick but non renewable versus slower but renewable.
I don't know, just throwing an idea out xD

As you can tell, I am in favour of a mortal world. We could even have a set of religious zealots who want to destroy anyone and anything with any connection to the demonic beings that seek to ravage their world. The mortal world, much to the chagrin of the poor souls who live there, happens to be the one and only source of valuable materials the Dark ones need to expand, and like the colonists to the Indians the gods aren't too interested in sharing it. The Gods could also do another Daedric prince thing and have champions, or... Well, anything I guess. I feel we would need a lot of clear limits on our godly powers.
I am not too sure what I would want to be the Dark God of... Maybe I can be the god of uncertainty xD