Possible candidates imo:
Leiter, Jr - Cubs could easily keep him too but having a career year and 32 years old...might be better to sell high even if his improvement is sustainable
Hendricks - Final year of deal but team option if he pitches well. All depends on how well he pitches. If he pitches well he becomes a somewhat attractive piece. Otherwise, Cubs will probably just keep him and run out the clock.
Merryweather - Similar to Leiter....he'll be 32 in October and having a solid enough season to potentially get something of use for him
Smyly - Less likely because he has another year on his deal and to TT's point I don't think the Cubs are looking to take a huge step back, but if you get a crazy offer from a SP needy team and it includes MLB or near MLB players, you at least consider it.
Stroman - everyones talked about this enough
Gomes - highly regarded pitch framer having a solid hitting season is bound to have some value for a team needing catching help, though the same questions that Contreras had about adding a catcher mid-season is a factor, plus the Cubs probably plan to have him in a similar role next year.
Mancini - talked about enough, your standard extra low cost bat for a contender. Was in the same role last year with Houston
Mastrobuoni/Tauchman/Madrigal - I guess? Don't really have any strong ties to them and if a team is looking for a bat, cya. Madrigal is probably better keeping and giving regular PT to see if you can get anything from him long term but he's very limited by having an everyday starter at 2B and SS.
Bellinger - covered by TT and I agree with his take
Yeah, after typing all this up, there's definitely not a ton there. I think you shop most of those guys, if you get a strong offer you take it. You definitely shop Stroman to see if a SP needy team will throw something crazy at you. Would be stupid to not at least shop him.