The fact that Aerith and Zack's deaths are seen as so sacrosanct is precisely the reason they're being toyed with now. If they left them alone, that's a lion's share of "FFVII" being left safely frozen in materia. The protection of what came before in terms of the world returning to normal, is a "stake" for the story, in a twisted sense of the narrative. It's a paradoxical challenge being rubbed right in our faces. If their intent is to tell a dynamic story that's meant to go beyond the borders of what came before, with a reset of expectations for the audience... They have to show willingness to challenge their own performative limits, and the limits of the player.
Does that mean they must ultimately completely undo what they've done thematically with their story/characters in that regard?
No, of course not, and I don't think that's what they plan to do, either. But their willingness to challenge just how far FFVII is willing to stay FFVII makes it that much more interesting and opens the door to at least, explore something new and different with the characters and the themes presented thus far. They're in essence, unwilling actors in a repeat of their previous adventure, half-aware of how things may go or were "meant" to go. That can dive deep into some interesting interactions and feelings, and some places that some might not wish to go. However, if you're gonna open pandora's box, that's sorta what happens. At the very least, they can tease and examine these things without ultimately shattering what came before. That's the whole point of this. And Zack being paradoxically present for us yet still deceased in Cloud's past and present, can offer up an interesting collision between illusion, reality, and the wish for a future that ultimately
cannot exist. There's plenty that can be done, it's just a matter of if the writer's are going to actually do it, or just make it some fake and escapist Hallmark happy ending that would be an ultimate betrayal of what those deaths were meant to represent. I really don't see them doing that, and I hope to ultimately be proven right.
...Just like I was with the Silver-Haired Rainbow Whispers.