<I don't think we'll get DIRECT Darkness involvement, but it's entirely possible that the armada's technology will be introduced. Clearly Bungie's trying to keep identity and motives a secret, but technology and (human-crossed) history is fair game. We'll be getting a -blam!--ton of hints to be sure. As for the Hive and Vex, I'm thinking their war will reignite. Whatever the Hive are doing brought back Zydron, and there is now a Vex Gate on the Moon, presumably our path into the Black Garden, which has been flung into space. The Black Garden wasn't made by the Vex, and it's possible that its original creator has moved it as bait. The Vex get stronger by being in the Garden, so it'd make sense if they made a resurgence. The Vex will spread to all they can and push back the Hive, only to be met by the Taken. If we cripple the Taken and kill Savathun, the Vex will win the war. They will be unstoppable (except to us) until the Darkness arrives and pushes them back harder than the Taken ever could.>
All this nightmare and vex involvement has me questioning if the vault is involved somehow. Hive magic with vex tech could have some crazy consequences. I do wish we will go back to the vault or talk about it somehow in the future because it was the best lore thing destiny has in my opinion.
I think the Vex could definitely have a role in the moon storyline and I wouldn’t dismiss the possibility that a certain hydra might be behind the Nightmares. I still don’t really see Savathûn in all of this, but that’s probably the intention. I don’t know if there will be war between Hive and Vex, Taken specifically seem more likely to me. Savathûn and Quria love to send them to infiltrate their networks, I have a feeling we might see more of that. The Vex might also be this coming years secondary storyline, like Shin’s story was for Forsaken and the annual pass. They are seemingly more tied to the Season of Undying than Shadowkeep itself.