Kingdom Hearts III Speculation

At the end of KH:0.2 "A Fragmentary Passage" you basically get a peek at some of the beginning scenes of KH3, where Yen Sid basically tells Sora "You're level 1 again. Go grind out some levels in the Olympus Colosseum." It lets us know that the predictable structure remains true. The Colosseum will be on the far side of the first ring of explorable worlds, which means a choice of one of two worlds before you get there. 

Overall the goal is to unlock Sora's power of waking so he can restore the hearts connected to him: Roxas, Namine, Xion, Aqua, Terra, and Ven.

The pattern laid down in prior games is like follows:

• Starting World - (Destiny Isles/Twilight Town)
• Hub World - (Traverse Town/Hollow Bastion)
• First Circle - Three worlds, so initially you choose one of two to access the third on the far side. These worlds gradually introduce game elements and the thematic elements.
• KH1: < Olympus Coliseum, > Wonderland, Deep Jungle
• KH2: < Beast's Castle, > Land of Dragons, Olympus Coliseum
• Return to Hub World
• Sub World(s) (100 Acre Woods) 
• Side Plot World (KH2: Disney Castle/Timeless River)
• Optional Side World (KH2: Atlantica)
• Second Circle - Four/Five Worlds, starting with a bottleneck, then a choice of two, and a final world on the far side. In the case of KH1, Neverland doesn't appear until the other worlds are visited. Also, Monstro can appear between the worlds on either side of Agrabah.
• KH1: Agrabah, Monstro, Atlantica, Halloween Town, Neverland
• KH2: Port Royal, < Halloween Town, > Agrabah, Pride Lands
• Return to Hub World - 
• Sub Plot World - (KH2: Space Paranoids)
• Revisit Previous Worlds
• Plot World - Hollow Bastion
• Final World - End of the World, World That Never Was

Using that as a foundation, I will now go into my predictions about the structure based on the worlds that have been announced. To satisfy fans, it needs to have a mixture of familiar and new elements. Barring last minute surprises, those seen in the trailers should be all of the main worlds you visit.

• Starting World - Radiant Garden?
• Hub World - Twilight Town
• First Circle
• Kingdom of Corona
• Toy Box
• Olympus
• Return to Hub World
• Sub World - 100 Acre Woods
• Side Plot World - Disney Town
• Side World - Beast's Castle
• Second Circle
• Arendelle
• The Caribbean
• Monstropolis
• San Fransokyo
• Plot World - Hollow Bastion
• Sub Plot World - The Dark World
• Revisit Previous Worlds
• Return to Hub World
• Plot World - Castle Oblivion
• Final World - Keyblade Graveyard

As a prologue, the starting area is a training zone, so I would not be surprised if this involved Kairi's training with Merlin... in Radiant Garden (formerly Hollow Bastion). Her training partners include Lea and Max (Goofy's son). Since starting areas in prior games have been destroyed or locked off for a while, I expect something to happen in which Radiant Garden is captured, probably by Vanitas, Maleficent and Pete.

Vanitas undoes the lock on the world, allowing heartless to swarm the world’s heart, and send HB into the darkness. Riku and Mickey use the opportunity to enter the world of darkness. Kairi awakens in Traverse Town ala KH1. She is greeted by Rinoa who offers to take her to safety.
 
Meanwhile at Twilight Town. Sora, Donald, and Goofy go to the Mysterious Tower... Yen Sid tells Sora to train with Hercules at the Olympus Coliseum. The first circle of worlds is opened, the Coliseum being on the far side.

When you arrive at Olympus, you start at the bottom and climb to the top where the Coliseum is located. You encounter Maleficent and Pete looking for the mysterious black box from Unchained X. Your training with Herc is cut short by more pressing matters, most likely whatever happened to Radiant Garden and Kairi.

Chip and Dale inform Sora that Lea showed up at the Disney Castle with news of the fall of Radiant Garden. Lea brings Kairi and Max to rendezvous with everyone in Twilight Town. This leads to opening the second ring of worlds.

As an aside, Disney Town will be available at this point and perhaps, Beast's Castle, this game's musical feature as was Atlantica in KH2. Yes, he becomes a beast again. There has to be a fight with Gaston, so the prince has to be the Beast, dang it.

After completing the second ring of worlds, Chip and Dale notify Sora and Co, that Hollow Bastion was reappeared. It is during the exploration here that Sora is sent to the dark world, where he meets Aqua, seemingly taken over by darkness, but truly only having golden eyes from long exposure to the dark world (like Ansem the Wise).
Following her, they leave the dark world together. Sora unlocks his power of waking, and this sets them on the next leg of their journey, affectionately referred to as the World Retread Tour. In Davy Jones Locker, Sora manages to bring back Xion, When revisiting Arendale, Namine is restored.

Reuinion - After revisiting all the worlds, Sora travels to Twilight Town to wake Roxas from within his own heart (perhaps using the simulated version). Outside the mansion, Ansem and Xemnas confront Sora. Xion and Namine allow Kairi to dive into Sora's heart and retrieve Roxas. Afterwards you can see Squall reunite with Rinoa there, after a fight with Seifer.

You will then be able to access Castle Oblivion and wake Ventus, which also restores The World of Departure. This in turn opens the way to the Keyblade Graveyard. It is at this point the black box is opened, and the final battle begins. The seven lights and thirteen darknesses create the true keyblade to... fix it all up there.

At this point your guess is as good as mine. Hopefully there will be surprises along the way. I might be totally wrong, and would be fine with that.