I am liking this idea for limited imp summoning. It means we get to have the flexibility of being a Keeper while at the same time still requiring the collection of human resources. We can say that the amount of imps you can summon is limited by your power, and you gain power by having slaves, or something like that.

EDIT: Ah, how about at the start, you get to summon up to 5 imps. Then you get an extra imp for each 5 slaves you capture, until you reach 100 imps, which can be the maximum. This means starting is not difficult, because you get some workers straight away, but as you progress the imps will recede into a managerial role (which will be important, because you don't want to waste your own time managing your slaves personally), rather than a worker role. The reason behind this can be that the Void here is difficult to access.