Nibelheim to Nibelheim, probably. If we're lucky.
That's my guess as well. They will probably expand and add more content in between things.
Nibelheim Flashback
Kalm
Chocobo Farm
Marshlands
Mythril Mine
Yuffie
Condor (???)
Junon
Shinra Cargo Ship
Costa Del Sol
Mt. Corel
The Gold Saucer
Corel Prison
Gongaga
Cosmo Canyon
Nibelheim
My prediction is that Sephiroth will be a guest character like RED XIII was in PART 1 or a party member that we wont be able to control, but give commands to like Sonon in INTERmission. Sephiroth might be used to introduced the game mechanics to the players. Combination attacks and such.
The first new character introduced into our party will be Red XIII, he will most likely be playable from Kalm.
Yuffie will follow after we cross the Marshlands and Mythril Mine, just like in the OG. She was last seen riding a Chocobo so she will have no problem crossing the Marshlands and would make sense that she is one day ahead of us as Cloud & Co. was busy infiltrating Shinra and fighting Destiny while Yuffie probably left Midgar right after Sector 7 Collapse.
Then, we have Cait Sith in Gold Saucer.
I can't predict if we will get any new guest characters throughout PART 2 since they might or might not add any new characters to the series or new story segments for existing characters. Though I am sure that one fan favorite character, Vincent Valentine will be a guest character at the end of PART 2. He will probably help us fight the final boss.
So if PART 2 is from Nibelheim (Flashback) to Nibelheim, then we will get at the least 3(2) new playable characters making it total of 7 playable characters and 2 new guest characters.
As for the where the game will end, I predict it will end with Nibelheim or the Cave near Nibel Reactor.
I suspect that if the game ends in Nibelheim then we will fight a new Jenova form in the Shinra Mansion when meeting Sephiroth again . OR that the final boss will either be the Dragon (which I think not) or the Materia Keeper boss inside the cave near Nibel Reactor and the game will end there.