The ending of Rebirth was a bit less confusing this time, but it's still far too ambiguous with the way it combines a bunch of visual cues that all imply different things (static, green glow, white light, rainbow glow, the tear in the sky). Based on my
previous list of theories, I'm now leaning towards a combination of theories 1 and 2, that it's a living alternate reality version of Zack in the final battle, but the Aerith who appears in the final battle is her spirit. As for why only Cloud can see Aerith in the post-final-battle scenes, I'm pretty sure now that it's theory 2. I can't shake the concern about theory 4 though, that it's being left intentionally ambigious because they haven't decided how to wrap up the story in the next game yet.