It's still very early and thanks to the expanded playoffs, the Cubs are still only 2 games out of a wild card spot. But, another 4 weeks like the 4 weeks that just went by and it'll be time to start thinking about selling at the deadline. Again.
Bellinger - He has a mutual option for $12.5, which he'll decline if he has any sort of season at all. He's been a nice story despite his recent slump. If he can keep his OPS in the .800s while continuing to play Gold Glove defense, he'll be the best CF on the market. He could net a team's top 10 prospect.
Barnhart - He has a player option for $3.5 next year that he will pick up. He looks to be becoming extraneous with Amaya's return to health and nice season thus far. Probably would only net a Rule 5 eligible non-prospect.
Wisdom - Hear me out. A guy trapped in MLB's late-bloomer contractual Hell. He's 31. He's got massive power and a great arm at 3rd. He makes inconsistent contact and strikes out a ton. He's still on a rookie contract and only gets to his first year of arbitration next year. So he's cheap with 3 more years of team control. I don't think the Cubs see him as part of their future core and he's certainly not the kind of player you'd extend, but absolutely is the type of guy who could be the 7th or 8th best bat on a championship caliber team. I think he'd be more valuable to someone else than to the Cubs and might net a relatively nice multi-prospect haul. He'd be a perfect fit for the Phillies or Yankees.
Stroman - He's got a $20 million player option next year that most guys would decline given the season he's having thus far. But Stroman is a bit of an odd duck who does things his own way, and has made it clear that he wants to stay in Chicago. I'm sure he'd like the Cubs to negotiate a multi-year extension, but I'm not sure that's the Cubs' plan for a guy that just turned 32 and a team that really likes its young pitching coming up. He'd certainly net at least a team's top 10 prospect.
Smyly - He's got a $8.5 million player option that he'd be insane to pick up at this point. Likely looking at a multi-year contract in the $12-15 million per year if he can stay relatively healthy. Given the cheap contract he's on and the year he's having, he could be worth as much on the trade market as Stroman is. That said, if the Cubs weren't to trade him, he's almost the ideal QO candidate.
Hendricks - He's got a team option that the Cubs won't pick up. He's been bad/hurt for 2+ seasons now. He'll be coming off rehab soon and will have until the trade deadline to establish some sort of value for a trade partner. It would be great if he could be back to his old self for one more ride with the Cubs before netting them a mid-level prospect of some sort.
Boxburger and Fulmer will both be free agents next year. (Boxburger has a mutual option that won't be picked up.) They've been utter disasters and look more like DFA candidates than trade chips.