Something that's kind of been bugging me; The doors that the Guardians open up to snipe the Gatekeeper, do they remain open throughout the fight, even after Atheon's killed? I'm thinking that as long as that door's open, you could leave after killing Atheon and check out that far away portal frame. We know that the past and future portals are in Atheon's domain, but nothing is said about that one in the far back just before Atheon's domain.