Retribution

tales from a disgruntled fired Bungie employee
s17 - eramis becomes unfrozen and is revealed to be a disciple and sieges the deep stone crypt while Ana is there. Season revolves around saving her. Epilogue involves exo rasputin breaking out and helping us kill eramis before she freezes and shatters him. Ending cutscene is of a ghost finding him.
s18 - the Leviathan reappears and the Vanguard sends troops to deal with it. It's revealed it's become the glykon but filled with hive AND scorn. Immaru battles with us for control over the Leviathan as it could help him. Epilogue is us finding Calus, a disciple, and killing him. Ending cutscene is of him warning us about something (about the black fleet maybe?)
s19 - As the entire Black Fleet armada is making its move, Immaru manages to revive Savathun who then helps Mara lift the curse using the last Ahamkara. Season revolves around it going rogue and us having to kill it. Epilogue is protecting the Dreaming City from the Black Fleet. In the end, it is destroyed.
It's currently planned for the Dreaming City, Last Wish, and all of Shadowkeep to be vaulted. To make up for the removal of 2 a raids, its planned an old d2 raid returns (either Levi or Scourge) during Lightfall.
In Lightfall, the Last City is destroyed and we move to space. Zavala dies. Space capital ships are introduced in order to have a safe space to be. Drifter will help evacuate citizens while the new Vanguard stays. Expansion is centered around the Black Fleet having control of planets and us having to fight back against them. In the end, we kill the Witness and stop the invasion.
It's currently planned for it to cost $50 with 10 missions, a new destination (30 loading zones), a new raid, 6 new exotics, and more.
Crossbows will come at this time however I'm unsure if it'll be just the bow or a new weapon archetype.
I do not know much about the seasons after Lightfall. Apparently hearing "season of the plague" but dont know.
All I really know about The Final Shape is that it's effectively an all-out warfare. Cabal, humanity, etc all basically fight for the right to be the final shape. From what I heard from a friend in another department they're planning on killing off the cabal and most of the fallen, as well as introduce a new type of hive enemy (not like lightbearer hive).
As for after Final Shape, based on what I've heard from friends and from what I've worked on myself, it seems like the next saga will have to do with the Vex and new species as well as expanding to new universes (not necessarily leaving Sol though).

Lightfall raid takes place inside of a broken, split Traveler as we attempt to heal it. Things go wrong.
TFS raid sounds like it takes place in a new destination? Looking at the concept art for it, it doesn't look familiar. It will introduce a new enemy (the "zorpalods" which will play a bigger role post final shape).

Iron Banner will also be removed by Lightfall and replaced with something else, don't remember what though.