Each border area is a chapter sized chunk of story. The yellow areas are questionably mandatory, as in, probably will be mandatory. Here's my current thoughts about how the Remake's brand of linearity will continue onto the world map:
Chapter 19: Midgar Area. I covered this area as optional because as many have pointed out, kicking off Part 2 mid-flashback is the narratively stronger choice. Leaving this area on the cutting room floor also allows the illusion of a greater distance between Midgar and Kalm, starting off the "world exploration" with a bit of immersive bigness (think the distance between Rabanastre and the Estersands zone in FFXII).
Chapter 20: Kalm Flashback. I think they can fit the Kalm Flashback into a single chapter if they don't bloat it to set up the Part 2 time travel circus. It's likely they'll do both. No, Genesis won't be in it, but Zack will be. The central conceit of the story relies on the player already knowing all the spoilers already, after all.
Chapter 21: Grasslands. This region is Remake's first "open-world" explorable zone and will strive for the same feeling of scope as the Calm Lands in FFX, Pulse in FFXIII, or the open road in FFXV. It's probable that the swamp will be combined with this zone. It's improbable that we'll have access to the east coast of this zone - more likely we'll be invisibly gated from endless fields stretching east, the second illusion of bigness.
Chapter 22: Mythril Mines. I mentioned this in another post but this will basically be the exact same as Chapter 5 in Part 1, a pure dungeon crawl of a chapter, maybe some splitting the party, falling through holes to get to the next area, and lots of people falling off things and other people grabbing their wrists in a pantomime of stakes. Turks are the final boss.
Chapter 23: Fort Condor. Here's where I think we'll get the next development of Neu-AVALANCHE, as here is an anti-Shinra faction you can join and support. If the writers haven't forgotten about Wedge and Biggs, this would be their opportunity to make a grand entrance. I would totally love for this region to be entirely optional but more than likely it will be mandatory, and its possible that acquiring Yuffie will become integrated into this chapter as well.
Chapter 24: Junon. Based on the length and variety of Chapter 4, I reason that the approach to Junon all the way through the parade and getting on the boat
could be a single chapter. Could be two, depends if this section of the game is interrupted by Nomura shit, or if acquiring Yuffie is integrated into the Junon story arc (as it was in my novelization /shamelessplug). The region between Fort Condor zone and the Junon fields will be another wide and intraversible border image, and you won't be able to backtrack (until Chapter 29)
Chapter 25: Cargo Ship. This will be fairly straightforward, won't it? This would be roughly Part 2's Chapter 7, with Jenova-BIRTH replacing Airbuster as the mid-game difficulty spike. Expect more Sephiroth. If Junon is split into two chapters, this might be the latter half of the second chapter, like the Whisper invasion was part of Chapter 4.
Chapter 26: Mount Corel. This will include both Costa del Sol and the Mt. Corel dungeon. Costa del Sol
won't be a sidequest hub because the next chapter obviously will be.
Chapter 27: The Gold Saucer. This chapter will begin when you arrive in North Corel, and end when you are thrown into the Corel Prison. This is going to be the funhouse chapter, as Wall Market was for Part 1. It will be everyone's favorite chapter. You'll probably have to fight robots in battle square while trying to escape capture.
Chapter 28: Corel Prison. Since this is Barret's arc, I don't think they're going to go too off the rails for this section. Dyne fight and the chocobo race comprising the "boss" elements of this chapter.
Chapter 29: Buggy! The West Continent will open up in a big way, and this will be the first proper time you can revisit areas from prior chapters, including the East Continent. I am hoping that Gongaga remains optional, but it's possible we'll be funneled through it or maybe they'll even bloat it into its own chapter. Anyway, the buggy chapter will conclude with the buggy breaking down and a short chasm dungeon leading to...
Chapter 30: Cosmo Canyon. Both the town and the Cave of the Gi will be included in this chapter, and if they don't nail this I'm gonna cry.
Chapter 31: Nibelheim. Will the main character of Dirge of Cerberus be optional? Hmm... No. The popular consensus is that this will be the end of Part 2... and indeed, I think they can (and should) expand this important location to make it more impactful... but we're only about a dozen chapters beyond Part 1.
Chapter 32: Mount Nibel. Similar to Chapter 15, this will be an epic-level dungeon chapter, with lots of action sequences. The Materia Keeper, like Valkyrie, will be recurring throughout the chapter.
Chapter 33: The Captain. We acquire our last party member here as well as the Tiny Bronco. Unless this serves as the end of Part 2, this part will be played rather straight, with Palmer serving as an oddly difficult boss fight (maybe the haywire Tiny Bronco will be an environmental hazard like the electric rails from the Crab boss in Part 1).
Chapter 34: Wutai. Another funhouse chapter. Probably not optional, with repairs on the Bronco keeping the party marooned on this island until getting their materia back. In my novelization, this sequence was the beginning of Book 4, but I don't think this would be a good way to start a Part 3 (despite the consideration that it hard-resets your materia inventory lol).
Chapter 35: The Keystone. Aka the Gold Saucer date chapter. This chapter is actually a proper open world chapter, as the Bronco essentially gives us access to every zone leading up to this point, which is a big reason I think we'd still be in Part 2 at this point. Another I think we're still in Part 2 is that it allows your choices to influence the date without having to rely on save states from prior games. This chapter could extend beyond the Gold Saucer, all the way to the entrance to the Temple.
Chapter 36: The Temple of the Ancients. Well we're now 18 chapters post-Part 1. It could be the finale of Part 2, but the elephant in the room is that the next five chapters place Aeris's death either too close to a split point, and call me precious (you'd be right) but I think it's important that she get axed in the
middle of a game -- that's what so shocking about it. As for the Temple itself, I'm not holding my breath for this dungeon, it will be absolutely chock full of KH dialogue and Aeris death portents. I'm already rolling my eyes just thinking about it. The likelihood of it being more than one chapter depends on where in the narrative it falls -- if it's the finale, it might even be three.
Chapter 37: The Sleeping Forest. This is another opportunity to do the sidequests from Chapter 35, and an acrophies or something will be the big forest boss before the Coral Valley and the doom of our heroine. This will be a real Chapter 6, y'know? It could also be combined with the next section, with the dungeons not being overly expanded.
Chapter 38: Coral Valley. I think there are precious few left who think the end of Disc 1 is a good ending place for Part 2, and I don't need to tell you what's included here.
Chapter 39: The Great Glacier. At first I thought that Icicle Inn might be it's own chapter, but it's more likely to be combined with the Great Glacier dungeon, but with a slower paced beginning to get players to Icicle and Elena. I don't think Gast's videos are going to be optional anymore. Putting this chapter and the chapter preceding it in different games is inadvisable, as feeling Aeris's absence here is an essential aspect of her death's emotional impact.
Chapter 40: Gaia's Cliff. I can only see this as a straightforward dungeon and a dragon.
Chapter 41: Whirlwind Maze. Should be pretty straightforward but, like Temple, I am not gonna hold my breath. Structurally this is the best place to end the second of three acts, and since it ends up scattering the party and imprisoning our perspective character, this would be an ideal place to reset the inventory leading into the final game.
Chapter 42: Sapphire Weapon. If it's played straight, this would be a great last chapter of a part, or the great first chapter of a part, or just a great sequence mid-game. I really hope it is as faithful as possible, as its one of the sequences that created my love of Final Fantasy and one of the things that had me clamoring for a remake.
Chapter 43: The Huge Materia. You have the Highwind and the open world. Finding Cloud in Mideel is but a single cutscene, and the two huge materia missions could and should be combined into a single chapter, especially since you have the choice of which order to do them in. Perhaps Ultima Weapon is the final boss of this chapter, leading us neatly into...
Chapter 44: Inside Cloud's Mind. This is one of those sequences that they can't possibly fuck up and can't possibly get right. They're gonna bugger around in it with Zack and honour and make it more about Zack than Cloud, but the game has so far been so unambiguously CloTi that they can't not nail my most precious moments
Chapter 45/46: Underwater Reactor/Sending a Dream Into the Universe. I think these chapters could be run fairly straightforward, since each has its specific goal and resolution.
Chapter 47: The White Materia. This chapter will start after the group returns to the Highwind from space, and is kind of an open-world treasure hunt for the Key of the Ancients. I think that Lucrecia's cave is going to be a necessary step to discovering the Key of the Ancients, maybe the Gelnika as well. Diamond Weapon is the boss of this chapter.
Chapter 48: Return to Midgar. A straightforward action sequence that I can't see straying too far off the path. Maybe a nod to DoC here or Hojo being involved with whatever additions to the story have clung to the hull of this ship through the Nomuric Ocean and the Kita Sea. It's possible you could squeeze out another chapter (maybe more) out of this sequene, with the final confrontation with the Turks, rescuing Reeve from Shinra HQ, and Proud Clod being major story beats in their own right (this might also help inflate the final game's low chapter count).
Chapter 49: The Northern Crater. (most of) The final dungeon!
Chapter 50: The Planet's Core. Unable to take screenshots at this time.