He can opt out after 2018 to be a unrestricted free agent again. Heyward has already said he'll take less money to play with a good team. and with Harper likely being a ufa also in 2019, it's possible Heyward will opt out, re construct his contract with the Cubs, and the Cubs sign Harper. Heyward could still play RF, and put Harper in CF. either move Almora Jr to LF, with the possibility of trading Schwarber, Happ or Zobrist for pitching.
The Cubs are still looking to get a TV deal, i think after 2019, which will help bring more money in. but, after signing Yu to a six year deal, they may need to look elsewhere from within if they want to sign Harper.
Bryant just got a one year, $10 million deal via arbitration, and will also look to extend him as well. but it'll be hard with Bryant and Harper both have Scott Boras as their agent. along with Addison Russell, though as good as Russell is, he's not a super star like Bryant or Harper. Bryant's not a UFA until 2022, when he'll be 30. And Boras has already said that he's a cornerstone player for the Cubs, but you never know.
Harper said he's not focusing on 2019. he's focusing on playing hard this year and winning. of course he's thinking about 2019. playing well this year will help him solidify in getting the richest contract in sports history.