Certainly the ices. No need to go so far out for that though. Bennu likely has a fair bit of water as hydrates and there is a fair bit of water ice in Saturn's rings.

Don't be inner-solar-system-centric :) Once the frontier has been pushed back to the outer solar system, volatile KBO and Oort Cloud objects become the new frontier. We can imagine a distant future where humanity planet-hops from object to object, transforming these volatile rocks into vibrant cities each with their own culture.

Don't mean to be an inner-solar-system-centric, but we all have to respect the economics of deltaVee. I'll grant you that if we're harvesting reaction mass from the Oort, it may become moot, provided we have some way to accelerate it to high velocity. Solar thermal probably isn't practical, there probably aren't practical concentrations of fuel for fusion and certainly not fission. I wonder if Mercury has relatively easily mined fissile material?

What delta-V? I'm not suggesting it go anywhere too distant. In a few thousand years there will be trillions of sentient beings living in the Kuiper Belt and Oort Cloud, trading with each other and building lasting, self-sufficient civilizations.