Looking at the lore which dropped in WQ, we can guess that we will have a season focused around Rasputin, a season around Calus's return, and a season about Erimis.
My prediction is S17 will focus on us teaming up with Anna and Mithrax to enter Rasputins mindscape. There we will work to remove the Witness corruption and rebuild his mind. Meanwhile as the season progresses we will see Anna make progress on Rasputins physical exo body and the season will end with Rasputin being upload into the body. Also I predict we will see Arc 3.0 in S17 given the mind spear was a taste of arc 3.0, and skip grenades were nerfed. Also trespasser is arc and we know that will return. The dungeon that will launch will have us clear the warmind bunker under shaxxs castle.
S18 we will see Calus return as a disciple along with the leviathan and we will manage to kill him with the aid of Mara and Ciatal. Ciatal may die and come back as the 1st cabal guardian.
S19 the Witness will revive Erimis and she will become the next disciple. She will then lead the Witness forces on an assault of the last city as part of light fall.
I hope so , Arc has been my favorite class since my D1 beta striker shenanigans
And I would love for them to expand upon house of light, we still don't have a house of light emblem
One thing I disagree with is that Eramis will be a villain, she will either be an ally or neutral, she will remember the darkness betrayed her and chose us , I am hoping her partner comes back and she returns to being the Kell who made a home for the Eliksni where they can lay there head down without worrying about Hive or Guardians, the Kell that Variks thought could lead the Eliksni back to greatness
But we will see