The first major shake-up arrives December 4th with the Season of the Forge, which lasts through February. The centerpiece is the Black Armory, which revolves around accessing a previously forbidden enclave of weapon crafters and the gear that comes with it. This includes a new raid, a new exotic quest and new triumphs to complete. Most everyone else will have access to seasonal events like The Dawning (holidays) and Crimson Days (Valentine's Day), new rank and vanity rewards and triumphs. If you have Forsaken, you can also expect Light cap increases every three months.
After that Bungie is providing less detail, but there is still an inkling of what's coming next. The Season of the Drifter (March through May) unsurprisingly focuses on Gambit and the premium Joker's Wild pack, including new gear, the weekly Joker's Wild quest and bounties from the elusive vendor Xur. All players can expect more from the goodies from the winter plus a new, unnamed "spring event." The final (and currently unnamed) season from June to August will introduce the final pass-based pack, complete with another new raid and the mysterious "6-player matchmade activity."
While Bungie has provided roadmaps before, it's rare that they extend this far. The company clearly wants to send a message: it's addressing the mistakes from the first year of Destiny 2, when there were long lulls between add-ons that left gamers without much to do. You should have a steady stream of things to do in the Forsaken was, whether it's new missions or simply improving your character. That's necessary to some degree. Activision has not been shy about admitting that Destiny 2 sales have not met expectations, and a regular flow of new content might bring back people who'd otherwise left the fold.