Obviously at the end, we see requiem getting taken into a portal, dropping their guns in front of the sentinel artifact
When they come to, they’re in a darker version of Janus Towers, one where somehow the OG crew is present.
Here’s the thing, they didn’t just end up in the Janus Towers spawn room, they’re in a dark aether version contained INSIDE THE SENTINEL ARTIFACT
My two basis for this theory(besides what I listed, and the cutscene showing them enter the artifact)
At the end of Citadelle, Kraft warns the crew to be careful of the artifact, because it can consume them. Originally, one May of thought its power would corrupt them, but in reality he meant it genuinely would eat you, and that’s where BO7 takes place.
My second part of reasoning is a bit niche, but on The Tomb, when you enter the dark aether for the first time, grey will sometimes have a line saying
“Looking at the galaxy’s on the amulet, I wonder if this is taking place inside it”
While this might be grey cooking a schizo theory(twin sista!), it could also be a hint that inside the amulet is a dark aether nexus, and inside a sentinel artifact is perhaps a larger segment of the dark aether?
(Also could just be the sentinel artifact is simply a gateway to the dark aether of course, but mine sounds fun I think)