On the subject of Zack and Aerith asides, I agree they should be used sparingly, but I doubt we're not getting any.
However, to that end I think it would be quite fun to parrelelize Zack and Tifa's searches and delay him finding Aerith until Tifa finds Cloud in Mideel, and even then we don't see her.
Chapter 1 I think is all Icicle inn, starting with Gast's Videos, he can and/or can even be our tutorial section playable character like Zack in Rebirth. I think Ch1 can be the whole stretch from Icicle Inn to NC. As such, I think it'd be really cool to make Cloud not party leader from the start or right after it, and reduce him from party member to guest to out of the party entirely over the course of the chapter, concluding in the events of the northern crater. Cloud is lost, Tifa is Comatose, and here we finally get a Zack interlude. he hears a voice, and goes to chase it.
Chapter 2, Junon and Rocket Town. In Junon the Midgar Infantry turn a blind eye to let Tifa escape Palmer still comes and tries to take the ship, but it's the Highwind instead of the Bronco. Party beats his ass, reclaimed the Highwind, and we're off to -
Chapter 3, Wutai, part 1. Wutai is amping up for war against Shinra again, we have to convince them that Sephiroth is the true threat. We don't quite convince them, but we make some progress. Maybe Rufus gets exposed at this juncture, throwing both sides into a kerfluffle and giving the party time to do sidequests, grab a Magnus materia and fight and a WEAPON fight or two. We deal with Corneo here and while he doesn't die, we absolutely beat him senseless.
Chapter 4, Here's where we finally find Cloud in Mideel and Tifa leaves the party. I think this probably ought to be at the end of the chapter. I think most of the main quest here should be catching up with people we've helped. Now would be an excellent time to check in with Leslie, maybe he's made progress finding his love but there's a problem- like her health- we cannot solve for him immediately. A Chocobo racing short quest would also not go amiss here- not to conclude things, but move things forward and give Tifa her time on the bird.
Anyways, right after Tifa meets Comatose Cloud, we finally get out second Zack interlude, where he sees Aerith but we do not. We can even hang a lantern on things by him calling her an angel again.
Chapter 5 is Cid's time as party leader, which basically amounts to the Corel train sequence and rushing right back to Mideel. There's no particular reason for Cid to ride a Chocobo, but we'll figure out some time in case the player wants to.
Chapter 6 is all Lifestream, followed by Zack interlude explaining his escape from ShinRa- recreating the flashback from OG, essentially- to a still off screen Aerith. After his recounting we jump back to the highwind where we finish up with Cloud having explained roughly the same thing to the party.
After that I don't think we should get another interlude until right before the final battle, and then Zack and Aerith doing assists in the fight.
I could go into more detail about how I think they could restructure the plot to streamline existing plot beats while fitting in the new ones.