I'd say Rebirth is actually going to end when Cloud is lost in the Lifestream at the Northern Crater.
There is assloads to go through, but if its 3 parts, that would be the place thematically to end it. Everything is going to hell at that point, and we can open part 3 with Tifa captured, setting up the gathering party members part.
What is good for Remake is the opportunity to make things less vague about the world, its politics, and its cosmology.
Vincent and Yuffie will be neatly integrated into the plot this time, we'll likely learn more of a cohesive way Gaia works in Cosmo Canyon that fits into how in the original, humanity seems kinda screwed looking into the future at the end.
They're putting out all this stuff so things are just crystal clear without any ambiguity about Gaia, Jenova, Seph, Aerith, and just lots of stuff. While also milking things for all they're worth on the road to that.