Finally writing down some thoughts I had back in September:
From watching the various previews, it's clear that a lot of time was put into Sephiroth's animations and abilities. A bit too much for one chapter if you ask me.
It's often been theorized around here that Cloud will be the final boss of Rebirth. I want to invert that idea a bit. What I'm envisioning is that the player will control Cloud and Sephiroth as they fight against the party. In this way, the two will team up once again, just as they do at the start of the story.
The State of Play trailer ends with Sephiroth saying "the reunion - when worlds merge". Considering we have two worlds already, Beagle and Terrier, I don't think it's a stretch to say that he's attempting to merge them in some fashion that we have yet to understand.
Therefore, this hypothetical battle between Cloud and Sephiroth against the party could take place after this "reunion" has been initiated. Then, once Cloud and Sephiroth defeat everyone, Zack appears to turn the tide of battle in the party's favor. It sounds cheesy, I know, but Zack is definitely going to involved with the ending in some capacity. With his help the merging is prevented, but not without consequences.
So the big question, where does Aerith's fate lie within all of this? In Remake, "defying fate" is a new theme injected into the story, and the game does not shy away from showing her death in relation to it. The party defeated fate, and thus ensured the future is their own to define.
But no one truly owns the future. No matter how strong your willpower is, you can't control everything. Sometimes life is just unfair. In Aerith's case, she may still die needlessly. Maybe that's the angle the story will take. Her fate could also be tabled, left as a mystery until Part 3. I honestly don't know. I'm 50/50 on it, which is exactly where the storytellers want me to be.