Age and the last season before each player becomes a free agent...
Likely 2017 starting lineup with no trades or other acquisitions:
C: Contreras, 24, 2022
1B: Rizzo, 27, 2021
2B: Baez, 23, 2021
SS: Russell, 22, 2021
3B: Bryant, 24, 2021
LF: Schwarber, 23, 2021
CF: Heyward, 27, 2018 (Can opt out after 2018 or 2019 then under contract until 2023)
RF: Zobrist, 35, 2019
Almora (22, 2022) and Soler (24, 2020) are on the bench and could be eventual options to step in for Heyward and Zobrist. You also have Eloy Jimenez, 19 and possibly the top corner outfield prospect in the game, making his Major League debut in 2018, if not sooner.
There will be more churn in the pitching staff, but the excellent years from Lester (signed through 2020), Hendricks (2020), and Montgomery (2021) give the Cubs an excellent base to work with after Arrieta, Lackey, etc. become free agents after 2017.