Well, I haven't played OG in forever so I had to dig my guide out to remind me of all the stuff that happens.
Disc 2:
Chapter 1: Opening is still at Kalm with an entire chapter dedicated to Cloud's flashback of Nibelheim, expanding on his crush (if they don't include the bit about Cloud looking through Tifa's drawers I'm gonna be so disappointed! The drawers bit is also a good point to add some shipping and/or teasing), seeing his mother and just fleshing out the whole village, so the tragedy of what happens hits harder later and the shock when they find Nibelheim rebuilt hits even more.
Looking around Kalm, doing side quests and expanding that whole bit before the chocobo ranch, since leaving the village draws a clear line where a chapter should end. It also gives time for world building and the eventual reveal of the world map.
Maybe address some of Cloud's glitches and make a plan.
Chapter 2: the chocobo ranch and mythril mine. I honestly don't think it needs adding anything new besides the usual polish and cut scenes magic. Something new for this chapter though, would be exiting the mine, hitting a forest and running into Yuffie because the guide at this point mentions you can get her as an optional party member from here - and we know optional party members won't be a thing. We could also make a pass at Fort Condor, but not go in since that's an optional section and something I'd be happy to drop for other new content - and we go back there later anyway.
Chapter 3: Junon harbour and city + sneaking into the airport, Cloud's parade and getting on the boat. The save Priscilla minigame could be redone so we don't blow her up like a balloon, and there's other side quest options that could be added, too. Ofc more cut scenes, especially once they're on the boat to give a transition to the scene and then a boss fight with Jenova once we're on board.
Chapter 4: Costa del Sol + new content. No idea what, but that part of the game is campy af and definitely could be expanded. Maybe another date option or lead in for the optional dates?
Chapter 5: Mt Corel + Gold Saucer, but only briefly for the latter since we get dumped into the prison. Maybe a pass through to tease what we could have seen, but can't because plot.
Chapter 6: Corel prison + Barret's arc. Just expand the shit out of it. I wanna see more Barret back story.
Chapter 7: Chocobo race + Gongaga. Having the lighthearted bit separated from Barret's arc gives a breather and the proper respect to how serious his backstory is. Cut scenes all over the place for Gongaga. Lots of Zack & Clerith fan service, maybe some Zerith too.
Chapter 8: Cosmo Canyon/Gi cave + Red's arc. I wanna feel as choked up seeing the statue of his dad as I was in the OG.
Chapter 9: Nibelheim + Tifa's arc. More Zack fan service and Clotif. Shinra mansion + lost number battle. Vincent joins the party, non-optional.
Chapter 10: Mt Nibel. The whole thing could be expanded to fill an entire chapter no problem and the boss fight with the Materia Keeper would be lit. There's also chances to learn more about the planet and materia and stuff too.
Chapter 11: Rocket Town and Cid's intro. Palmer boss fight could still be comical af.
Chapter 12: Wutai and Yuffie's arc. We meet the Turks on vacay and see Reno dropkick Corneo off a mountain.
Chapter 13: the cat and the keystone + date night. Stealing the keystone could be a mission impossible style event. Cait Sith joins the party and something I'd like to see change is that he doesn't give the keystone to the Turks after, since we know he's Reeve and that feels weird for him to still be on Shinra's side after all the things we saw in Remake. We see the outcome of all the optional dating choices and get to play about in the Gold Saucer for a while.
Chapter 14: Temple of the Ancients + Aerith's resolve. That whole dungeon is gonna be killer remade, and I'd like to see more of Aerith's choices this time round, since it's debatable what she knows. Also, the dream sequence redone, although after the first one it feels a bit stale to have them talk not in person again.
Chapter 15: Bone village and digging side quest. Sleeping forest. Cloud being snarky about having to dig up shit is the goal here. Finally getting into the forest and going round in circles has a lot of scope for fun frustrating dialogue, bit like the stairs climb.
Chapter 16: City of the Ancients. Aerith lives (psych). Even looking at the guide to cross refence, I can't get over how stunning the OG did this scene. I do think the actual death is a bit weird, so remaking that to give it maximum impact could work. Her death in hi def would be perfection and reintroduce the themes that were softened by events in disc 1. I honestly can't see Square making her live unless it's a DLC extra fan service au thing.
End game on Aerith sinking into the lake, plus a voice over in a demoralised tone of voice to further add to the seriousness and melancholy tone of events.
If Square is aiming for even chapters for each game then there's two more chapters they can add that have other content. Ofc there's gonna be more Jenova and Sephiroth in those spaces and I hope we see more of Cloud's PTSD and it doesn't just get washed away because it's no longer as important as a plot device. More shipping. I'm a Tifa fan and Square spoiled us with content in 1, so I hope they keep things balanced for disc 2, too.