ArizonaPhil's take on 2019 Cubs payroll and free agent possibilities:
So the Cubs will be about $10M inder the 2019 Competitive Balance Tax ("Luxury Tax") threshold even if they sign no free-agents, and that's even after they decline the 2019 $20M option on Cole Hamels (which they almost certainly will). They would save about $5M AAV by non-tendering Addison Russell (which would get their AAV+PBC down to $190M), but otherwise there won't be much wiggle-room unless the Cubs are able to unload contracts in a trade.
It's possible that the Cubs will attempt to trade one or two arbitration-eligibles like Kyle Schwarber and/or Mike Montgomery for prospects, but even doing that won't save more than about another $5M AAV combined, and they probably couldn't get "A"-level prospects back for either player. And while Jose Quintana might seem like a trade-candidate because he is making $10.5M (presuming the Cubs pick-up his club option) in 2019, the AAV of his contract is only $4.45M because he signed his multi-year deal when he was still pre-arbitration, so there isn't much savings in 2019 payroll AAV if the Cubs trade Quintana.
Otherwise, unless they can find takers for Tyler Chatwood, Brandon Kintzler, Brian Duensing, and/or Drew Smyly (whose combined contracts are worth $25M AAV), or eat a large chunk of his salary ($23M AAV) and trade Heyward (who now can be traded - see comment below), the Cubs are pretty-much stuck payroll-wise going into the post- 2018 off-season.
And so unless they are OK with exceeding the CBT threshold (and suffer the penalties that go along with that), there is almost no way they can sign a free-agent like Bryce Harper or Manny Machado to a high-AAV contract this coming off-season, and in fact they probably couldn't even re-sign somebody like RHRP Jesse Chavez without exceeding the CBT threshold unless they non-tender Russell or trade Schwarber and Montgomery.