The Royals' trio of Michael Wacha (signed through '27), Kris Bubic (FA after this season), and Seth Lugo (signed through '27) should be available. Sandy Alcantara should also be on the market, although his season has been kinda eh. Freddy Peralta might also be available, but that depends on how delusional the Mets are.
In terms of who to give up, I don't think anyone on the Cubs or in their system is strictly off limits, but, putting prospects and young controllable guys into tiers, for Tier 1 I'd hope they do their damndest to hold onto Horton, Shaw, Ballesteros, Hartshorn, and Wing (in that order); for Tier 2 I'd want them to hold out on trading Pedro Ramirez, Jaxson Wiggins, and Jefferson Rojas (in that order) unless it's in a deal for a multi-year guy or Skubal; and for Tier 3, I'd be generally okay with any other prospect getting traded assuming fair value in the deal.