It's past midnight and mly thoughts are still processing this info, so forgive me for being a little incoherent. This is very interesting, and
@Golden Ear I'm jumping on your post becvause it helps me think.
Does he bear that burden? I mean, is it said somewhere? I think there's a metaphore there with him being death versus Cloud who represents life, as he (Sephiroth) is dead, and Cloud is alive.
Actually, that is a very good question. If you think about it, on a meta level, Cloud & co did save the Planet in the OG. 500 years laters you can hear laughters in the woods; this is the good ending, the one we're bound to be back to. It is also the legacy that Cloud and the others leave behind them. They've saved the world, literally, they allowed it to be reborn while it was dying. This was the plot of the OG, it's still the plot in Remake; Cloud has to save the world to allow it to be reborn, like a Phoenix.
On a metaphorical level, I think we can link this. Now does this have a more practical result in Remake, like the creation of another, new world that would be the result of Cloud's wish? I think we'll have to wait on this one.
Sephiroth is saying that the Planet (=his and Cloud's world) is going to become one with the dying universe, which means it'll die and its last energy will join that galaxy, in the great cycle of life-style, as it's supposed to. Remember though that in the OG, the Planet was also very close from dying.
But Sephiroth does not want to become one with the dying universe. That's why he relies on Cloud to create a new universe, because he's unable to fight this fate he does not want. He knows his first plan he made failed (duh) so now he's trying something new maybe.
I think it's the Planet that is about to die. They're far away in the future. At least that's what's been theorised since the beginning here, and we kind of all agree on that.
Sephiroth does not want to die. He's stuck on the Planet, unable, like Jenova, to cross between worlds to eat their life. So he's stayed until the end, but he still doesn't want to die, so he's manipulating Cloud, trying to get something new to maybe win this time and get the possibility to cross between worlds and not be stuck anymore. Something like that, probably.
He doesn't have any usage of it, it symbolises his near death. He obviously doesn't want it.
Now that's THE question no one really knows about. There are lots of theories about Zack and for the moment, I wouldn't bet on any. I am very curious as to how they're going to handle him. All I know is that the OG enjoyed sending you in one (mis)direction only to catch you unguarded, so I think that's probably what's happening there too.
Well it's my personal theory that if there are multiple universes, then the Lifestream glue them together; they were indeed in the Lifestream before Sephiroth attacked them, and behind the Singularity the Whispers were duplicating the world. So I think there's something going on there that's most interesting but needs to be voiced by the devs for us to understand it.
I do think the idea behind the Cosmic Lifestream is not fanon? So bashically Planets become part of that Lifestream to be reborn at the end, just like life is reborn from the Planet's Lifestream, only on a cosmic level. But I'm not well versed in this, others will be able to clue you more about it.
We don't know if it's a time loop or not; nothing indicates the time loop. Yes there are visions of the future, easter eggs for fans, but if you think about it, there's not really any indication that this could be a time loop, especially since the Lifestream operates from the beginning to the end of the Planet, carrying the memories of the past, present and future. I do think that one of the possibilities is that dead!Aerith sent her younger self visions of the future in an attempt to prepare herself to fight Sephiroth.