There are a couple key players from last season that are free agents: Dexter Fowler and Aroldis Chapman. Both of these players proved to serve a crucial role in such a successful 2016 season, and the Cubs would love to have them back. Both the player's fates, however, are up in the air at this point.
As of right now, it is looking a lot like the Yankees want Chapman back pretty bad. The case of Chapman's scenario is somewhat rare; being such a dominant, tried and tested closer for so long, many teams don't know if they should shell out the money for a long term contract, or go all in and try their luck for one year. Very few teams have the money for a long term contract for a high-dollar closer. Those teams include the Yankees, the Dodgers, and the Cubs. Here is a great article that reflects that. I will say that it is pretty important to look back at the World Series and consider all of the confidence Joe Maddon put in Chapman as opposed to the rest of the bullpen.
Dexter Fowler, however, is a little bit of a different story. Dexter isn't as much of a high profile player as Chapman is. Although Fowler rejected the Cub's qualifying offer, I believe his heart is with the cubs, whereas Chapman may be more indifferent. It is a possibility that Fowler is looking for more of a long term contract with a salary the reflect how valuable he has been to the franchise the last two seasons. At this point, it is up to Tho and Jed to determine how badly they want him around considering the Cubs currently have the players to fill his spot.
No matter what happens, it is a pretty safe decision to put your faith into the Cub's front office. Theo Epstein has worked his magic with this team up until this point, and I am pretty sure that is not going to change.