Speculating is fun. Here's my attempt:
With FF7R in our hands, we have our best look yet on how future parts will be structured and what SE is capable of delivering with ~3 years of full development time. Here are some guidelines I'm going to follow in my speculation based on what was offered in part 1:
1. Part 2 will feature around 18 chapters (basically areas/maps)
2. Part 2 will feature around 3 "town areas" with side quests (analogous to Sector 7 slums, Sector 5 slums, and Wall Market)
3. All areas from the OG will return (This is the sited reason FF7R was split into parts, so that it's not an "abridged" version)
4. All minigames from the OG will return (FF7R embraces the OG's eccentricities)
5. Most importantly, a new mystery will be introduced and resolved in some form (analogous to the mystery of the Whispers)
I want to emphasize that last guideline. Think of the remake project as a crime series and each part is a season. Every episode in a season features a self-contained plot, but an ongoing mystery is also established early on (e.g. Whispers surrounding Cloud and Aerith at Loveless Avenue). This ongoing mystery presents itself to the main characters throughout the season (e.g. Whispers correct events). At the climax of the season, the main characters unravel the mystery but are consequently changed (e.g. defeating the Whispers and defying fate). The audience speculates how the change will affect the next season (e.g. this forum right now).
Serialized media often use this structure to make each installment stand on its own and engage the audience between parts. As such, I think part 2 will feature its own mystery to surprise players and help establish it as a full-fledged game.
Additionally, I don't think it's feasible to expect each town from the OG to be just one chapter in part 2. Towns seemed the most difficult areas for SE to develop in part 1 likely due to the amount of voice acting to record, event logic to debug, and data streaming to optimize. In part 1, each new area (or map basically) was considered its own chapter. A great example is the second bombing mission, which is split across three chapters (or three areas: Corkscrew Tunnel, beneath the plate, Mako Reactor 5). Stretching out the second bombing mission also benefited the narrative's pacing as it broke up the town areas (Sector 7 slums and Sector 5 slums), which feature a different style of play (side questing).
I've tried to consider all of this in my speculation: workload, mystery, and narrative pacing. Here's my list of chapters for part 2:
1 - Journey to Kalm (tutorial and exposition)
2 - Nibelheim Incident part 1 (pre-Jenova discovery)
3 - Nibelheim Incident part 2 (post-Jenova discovery)
4 - Kalm (town with side quests #1)
5 - Chocobo Farm and Midgar Zolom (chocobo riding minigame #1)
6 - Mythril Mines and Turks
7 - Fort Condor (real-time strategy minigame)
8 - Forest event with Yuffie
9 - Lower Junon (town with side quests #2, CPR minigame lol)
10 - Rufus' Welcoming Ceremony (parade minigames)
11 - Cargo ship
12 - Costa del Sol
13 - Mt. Corel
14 - North Corel (town with side quests #3)
15 - Gold Saucer (new date event, coliseum, minigame bonanza)
16 - Corel Prison (chocobo riding minigame #2)
17 - Gongaga part 1
18 - Gongaga part 2
Now for my theory as to what the new mystery could be to entice players. This is where I jump off the deep end. As I've said elsewhere, I'm thoroughly in the "Zack is Back" camp, hence the two part finale at Gongaga in my list. I speculate that Zack will also pursue Sephiroth throughout part 2, albeit ahead of the party. NPCs will talk of another SOLDIER 1st Class who helped out around town just a few days prior. "A charismatic man hitched a ride to cross the ocean just before you arrived" kind of thing. In the finale of part 2, Zack and the party's paths will finally cross at Gongaga, Sephiroth will appear for spooks, and Shinra will be around in full force as they excavate the old mako reactor. All the pieces necessary for a suitable finale.
But that's just my speculation! Sorry if I lost ya. At the very least I hope I provided some food for thought. My big takeaway is that I don't think part 2 will make it up to the Forgotten City or Northern Crater, not unless they implement a world map and reduce town sizes.
Furthermore, I think part 3 could end at the Northern Crater and part 4 will end, well, where the OG ended!
(Check my sig for my fanfics~) I've just been busy replaying this wild game. 
