I don't know why so many people suspect Aerith's death will be in part 2 - The party visits so many locations before Aerith's death, that it would at least need to be at the end of part 3.

Here's how I think it should pan out:

Part 2 - Kalm flashback to Nibelheim/Mt Nibel (Vincent as a guest like Red XIII was in part 1)
Part 3 - Rocket Town to City Of The Ancients
Part 4 - Ice continent to Final dungeon

These are the obvious narrative choices IMO. Part 2 would effectively open with Nibelheim (Kalm flashbacks) and also end with it (visiting Nibelheim for real). Part 3 would be the next step of the journey, possibly with some sea exploration added with the tiny bronco, and also Wutai being compulsary. And part 4, the final part, would both open and close with the northern continent.

I also think Cid should feature in part 2, during some scenes with Shinra maybe where he tries to get the space programme running or something. And then he doesn't join the party until part 3. This way, Cid would not miss out on part 2, thus every playable character would be introduced by part 2, but with Cid and Vincent not being actual party members until part 3. This would work best IMO.

As for new stories and stuff - I welcome them, AS LONG as they are not at the expense of the main story we all know. I don't want them to axe anything.