Currently, it is possible to find small bands of sentient critter-people in the caverns, which may be hostile or friendly to the dwarves. Hostile cavern civs are basically just another thing to kill in the caverns, but the friendly kind would hold more promise to give dwarves something to do besides just kill hostiles in the caverns and mine resources.
Unfortunately, they just sort of sit there, and let you steal their stuff.
In the grand tradition of Imperialism, why not give us more to do with native peoples whose lands we are invading than merely ignoring them and maybe stealing some of their food? We could be doing things like
stealing their children so that they can be "properly raised", or else paying them to kill the other things in the caverns for us and trade whatever beast carcasses to us. You could even arm them with armor to stand a chance of surviving some of those attempts.
Maybe a rift might split in their civ, where some want to keep the old ways alive, and others want to adopt the new ways of the outsiders with the thick armor to protect them from the hostiles around them, and the strange contraptions that rain magma upon their enemies.
Something I'd really like to see, though, is the ability to get a few of them to merge into your own society, and become part of dwarven culture.
Whether that means "Full citizenship" or "slavery"... well...
EDIT: For the purpose of making the full suggestion in one post:
Part of what I want to see is some sort of trading post and liason to the cavern civs that would let you interact with them diplomatically, economically, and potentially even culturally. You could have missionaries that try to "bring the light of civilization to the primitives", have hunters exploring the caverns who trade with cavern civs to get more pelts or the like, or just shove them off the land because you want it. (Just to go over the Spanish, French, and English ways of treating the Native Americans.)
Of course, while this all helps illustrate the options, this is also making cavern civs into
very obvious analogues to actual historical events and peoples, which needs to be avoided. For one thing, I know there's at least one actual Native American on these forums, and that person would probably not like the comparison to a fish-person who sits among piles of their belongings and just stares at walls all day. Fish-peoples, at least as currently implimented, really
are primitive.
To that end, though, we could have some more unique abilities that cavern civilizations might possess. They might, for instance, have some sort of reactions using some cave creature parts that might make useful chemicals or medicines that would be valuable to dwarves for trade. It would give your dwarves an obvoius incentive to keep them alive if there were some sort of medicine that were more effective than dwarven doctor's already woefully inept care. A tonic that raises the healing rate, for example, the way that soap is supposed to.