The "what change" is atheon died and the "left with out question" could refer to how every one just returns to orbit as soon as everyone runs around and picks up all the little drops. I think there is a reason why there is that long time between atheon dying and the loot screen coming up. There. May be something in that room, maybe to do with the sync plate and where atheon spawns but in the portals

Hmm... I really believe something happens after Atheon but that "pedestal" thing is confusing... What is considered a pedestal to Bungie? Maybe something must be triggered for something to spawn on there?

I'm not really sure what else there is to do after the kill... I've explored both portals, you get limited time because you do die eventually and there is nothing there I could find to activate... I've tried jumping across to the light at the back of Atheon spawn platform, I can almost make it there on my Hunter by dancing across but its still not quite enough to get there. I've also climbed the glass pyramid above Atheon spawn point to as high as I could with no success. Perhaps it is more of a team thing though rather than a single person? There are the 2 pillars on either side of where Atheon stands when you kill him that may possibly need some action?

Possibly... But going to where you blade danced, maybe someone could try to use the relic to cross the gap. They would need to make it fast but it may be worth a try.