Ah, now that's getting somewhere. A common resource, of course. Basics of population dynamics: if there is no competition over anything, then no struggles for survival. Human slaves is a good choice. The need for workers is universal to Keepers, so simply removing imps (which we can get away with easily since we've just hopped dimensions) brings that demand for humans in without having to change anything else. And if/when humans are sparse, Keepers will have to capture slaves from other Keepers.
Humans also need to be fed and housed and cared for, otherwise they die, which adds an extra layer to it. Unless you dedicate a large amount of infrastructure, resources, and workers into caring for your humans, your number of slaves would slowly diminish, demanding continual expansion. The willingness of your slaves is another factor too. If you take the guise of a benevolent overlord, then your human underlings would happily work for you and you would get maximum efficiency out of them. If, however, you overwork them, or if you've had to massacre all their family and friends in front of their eyes, they would be pretty poor at working, although you could probably compensate by just getting more. But, of course, if you get more, you have to feed more, which means you need to assign more as farmers. At this point the functions of demanding tribute (or forming trade alliances) become relevant. For example, you would be able to obtain food without having to expend your own workers. Or metals means you do not have to invest in a mine and miners. Etc.
This could work.
P.S. Cav, we roped you in because you were one of the originals and the Horsemen were your characters and we valued your input.
Next time, we need to figure out some plot which actually works well in the context of DK. Personally, I think the Ripper plan, while cool, on the whole lacked relevance to the individual Keeper and Dungeon. Something along the lines of those heroes and titans Trapezoid mentioned (but tweaked and fleshed out some more, of course. If its a good idea we can elaborate in PM), or something like an extension of what was happening with the Annointed of Caldor, would form a better plot for our Keepers to get tied up in, because it happens where they are not in some distant place.