The 40 men who brought the Cubs a 97-win regular season in 2015 won't all be back in 2016. Every team makes changes year-to-year in order to improve, and Theo & Co. will certainly do this with some of these players.
Some dates to remember: five days after the World Series ends, players can declare free agency. About a week after that, qualifying offers can be given to free agents (usually coinciding with the general managers' meetings, scheduled this year from November 9-12). Teams who want to add players for the Rule 5 draft must do so by November 20. The Rule 5 draft will take place the last day of the Winter Meetings, December 10.
Here's the contract status for 2016 (and beyond, if signed) of every player on the Cubs' 40-man roster as of October 25, listed in order of 2015 salary. In some cases the contracts are more detailed than listed here, but I'm doing this strictly for signed status, not dollar amounts or other contract clauses.
Jon Lester: signed through 2020, mutual option for 2021 ($10 million buyout)
Trevor Cahill: free agent
Miguel Montero: signed through 2017
Dan Haren: free agent, has announced retirement
Dexter Fowler: free agent
Jason Hammel: signed through 2016, club option for 2017 ($2 million buyout)
Austin Jackson: free agent
Fernando Rodney: free agent
Starlin Castro: signed through 2019, club option for 2020 ($1 million buyout)
Travis Wood: third year of arbitration
Anthony Rizzo: signed through 2019, club options for 2020 and 2021 ($2 million buyout for each)
Tommy Hunter: free agent
Jason Motte: free agent
Jake Arrieta: second year of arbitration
Jorge Soler: signed through 2020, but can opt out and into arbitration once eligible
Chris Denorfia: free agent
Pedro Strop: second year of arbitration
Chris Coghlan: third year of arbitration
David Ross: signed through 2016
Jacob Turner: first year of arbitration
Clayton Richard: free agent
Jonathan Herrera: free agent
Taylor Teagarden: free agent
Hector Rondon: first year of arbitration
Justin Grimm: first year of arbitration
Yoervis Medina, Tommy La Stella, Arismendy Alcantara, Kyle Hendricks, Matt Szczur, Dallas Beeler, Javier Baez, Quintin Berry, Kris Bryant, Carl Edwards Jr., Eric Jokisch, Zac Rosscup, Addison Russell, Kyle Schwarber and Christian Villanueva are not yet eligible for arbitration and thus can be renewed for 2016 at any salary the Cubs choose. It's possible that some of these players could get long-term contract extensions before the 2016 season begins; others could be non-tendered, as could some of the arb-eligible players.
The Cubs have contract obligations to two players not on the 40-man roster. They owe Edwin Jackson $11 million in 2016, and they owe Gerardo Concepcion $1.2 million.
As you can see, there will likely be considerable changes to the Cubs' roster for next year. 10 players are free agents. Some of them will probably be back; others won't. As noted above, some current players not free agents could be non-tendered and won't return. The Cubs thus will have considerable space for protecting players from the Rule 5 draft, if they so choose.