The game (Part 3) could potentially go on for even longer than what we normally played in the OG game. Is this something similar to how your interpreting or imagining it?
Nomura told Famitsu in July that “The reason we didn’t announce how many parts there would be until now was because we were considering whether it would be a trilogy or a two-part series...”
That would imply to me that the rest of the original game's story after Midgar
almost fit into one more game. Which is wild to think about.
By splitting the remaining story into two parts, we can expect events to be expanded, but it also allows room for new content. I think that the Remake Project's mysteries will continue to build until the very end and, as you suggested, the finale will receive the greatest expansion.
As for how I think they'll approach multiple universes in their storytelling (I prefer to say "universes" instead of "timelines" to avoid confusion with time travel), I think they'll confirm the existence of a multiverse, but we'll really only follow the events of the Beagle and Terrier universes as they're the most important.
I find the question of "how is there more than one planet, more than one sun, more than one lifestream" to be an interesting one. I wonder, did the Spider-Verse face a similar question at some point? I have no clue if the Remake Project will provide a lore reason for why there's multiple universes, or if it'll just be presented a fact with no further elaboration, but I do know how
I'd approach it.
(The rest of this post is just stuff I've said here before)
There's a cyclical theme of "life and death" running throughout FFVII. The original game presented this on a planetary scale, Dirge of Cerberus expanded it to include the cosmos, and I think the Remake Project will stretch it further to include reality itself. Much like how humans are part of the planet, and planets are part of the universe, I think it'll be revealed that the universe is part of something even greater.
And like humans and planets, universes are born at different times. This could be how the party's escape from Midgar in December is happening simultaneously with Zack's last stand in September. I don't know why the Whispers are working to ensure the big events of the original game are followed in each universe, but that's the conclusion I've come to after reviewing Remake many, many times. Even the Ultimania presents the goal of the Whispers as one of four new "mysteries" for players to ponder. Only time will tell.
Finally, and I've shared this before and will continue to do so, the Material Ultimania names the universes shown during the Edge of Creation scene as the angelic "dying universe" and red "universe that will be born" (translation by
TurquoiseHammer). We can see that cyclical theme of "life and death" right there. For me, that was the final piece of evidence I needed to be truly confident in my theory.