We don't know for sure what they are talking about in these working-level talks, but we have an idea, right?

Yes,... different experts have pointed to a number of possibilities, and one of them, is the "price" for North Korea's denuclearization.
The experts I spoke to believe the big framework for the regime's complete, verifiable, irreversible denuclearization and a security guarantee has already been agreed upon.
But, the detailed roadmap, and the reward for each stage may have not yet been settled, as that would require much more work and further negotiations.

" A peace treaty.... possibly diplomatic ties, the lifting of sanctions and economic support. These are all things the two sides can exchange, but when, and to what extent,... has yet to be decided."

As you'd imagine, this is no simple task, as experts say there's not much President Trump can offer the North in the short-term.
It's reported the U.S. wants a "front-loaded" agreement, where the North has to first give up all its ready-to-use nuclear warheads.
Experts say that while Trump wants big things from the North in the initial stages of its denuclearization, he doesn't actually have the cards to make that happen at the moment.
For example, lifting sanctions and settling diplomatic ties with the North is not a decision President Trump can make alone.
He'd require congressional approval for both... and it would take time.

The range of the agenda could also still be up for discussion of these working-level talks.

"In drawing up a roadmap to denuclearization, the U.S. could want more than just nuclear warheads and ICBMs. The discussion on whether to include other weapons of mass destruction, like biological weapons, may be another talking point between the two sides."