As mentioned, the overall plot is already known. We've been told that the game will conclude the "Xehanort Saga", which means that something huge has to go down at the end of the game. We know that Sora, Riku, Kairi, Terra, Ven, Aqua, Lea, and Mickey can all wield keyblades. However, we don't know who the seven guardians of light are. While it makes sense for Terra to be one of the seven, I'd say it's more likely that Lea is one of the seven guardians of light at this point, considering how much he's done to save Sora and others.
As far as the darknesses, I am completely lost. There are a few I have the utmost confidence in, given that they have already appeared: Xehanort, Young Xehanort, Braig, Isa, Xemnas, Ansem: Seeker of Darkness, and the possessed version of Terra known as Terranort. As for who the others could be, well here's a thought:
Riku was possessed by ASOD, which means that a form of Xehanort was present at the time of his existence. "Rikunort" as he was called could be brought back. That still leaves five different spots. All of the original Organization XIII members are gone except for Xemnas--who has been brought back from another time. It will be interesting to see what Tetsuya Nomura does to fill these spots.
As far as I'm concerned, there are going to be a few key moments that have to happen in the game:
1) Aqua will return to the Realm of Light after an absence that has simply been far too long. She has an extensive knowledge of Xehanort's past plans and has a strong connection to Terra and Ventus. She's going to meet/team up with Sora at some point.
2) Ventus needs to be woken up. Aqua knows where he's been hidden, and now we know how he can be woken up: Riku. Yes, conveniently, Riku has gained the ability to wake up people's hearts. He did it with Sora, and he should be able to do it with Ventus.
3) Something's gotta happen between Kairi and Sora. Thus far in the series, the creator has portrayed a huge connection between the two, but we have yet to see it fully mature. Now that the characters are going to be set as 18-year-olds (most likely), I think it's time to either establish them as friends or something more.
4) Xehanort must go. Ending the saga with something other than Xehanort's termination at either the Keyblade Graveyard, Kingdom Hearts, or the Destiny Islands wouldn't do the series justice. I think it makes the most sense for Sora to put an end to Xehanort in a big final clash.
I fully expect the same hack-and-slash style that has been present for the whole series. I also expect the elements used within the side games to appear in the third game. Ideally, we see the reaction commands from KHII, the command deck and command styles from BBS, and the environment interaction that was portrayed in 3D (videos are all below).
KHII Reaction Command Examples (watch at 8:20 for the best example)
I'd also like to see more interaction between Sora and his party members. It's been nice having Donald, Goofy, and others be in the party and act on their own, but I'd like to see more team-up attacks and whatnot. It's been a while since we've gotten the opportunity to have party members, so I hope they address this.
Since the creation of the latest game, Disney has been busy acquiring properties, as they now own Lucasfilm and Marvel Studios. I've seen many fans complain about the possibility of using these titles in the next game, but let's be real: if the goal is to get more dark and serious, you can't just stick to Disney. The series was very clever in its adaptation to Disney's "Pirates of the Caribbean" series, and I'd like to see more of that. I don't mean to make Sora fight in a Star Wars themed world, but I wouldn't mind seeing Marvel characters worked into the mix. They are cartoon-based and have back stories that can be easily woven into the Kingdom Hearts universe.
Please, for the love of all that is good in the world, INCLUDE PIXAR IN THIS GAME. Toy Story was initially developed into Kingdom Hearts II before being removed. Another world that needs to be included is The Jungle Book, which seemingly is removed last second in the development of every game. I have ideas listed below:
World of Heroes (Marvel-based)
Toy Haven (Toy Story)
Jungle Ruins (The Jungle Book)
Port Royal (Pirates of the Caribbean 2: Dead Man's Chest)
Tangled (no idea what to call this world, but this movie easily fits in plot-wise)
Sword and the Stone (Merlin's been pretty important thus far)
There are a lot of options that haven't worked out for whatever reason. Let's see a more serious game with world inclusion that goes beyond cartoon Disney.
Final Fantasy Characters
Despite their inclusion in the plot, Final Fantasy characters have been light on usage in the games thus far. I understand the desire to make the series about Sora and Disney, but let's get some FF characters in parties. I'd like to see Leon, Tidus, and Lightning make appearances in the series and be party members. At the very least, get Lightning into the plot, because her character in the Final Fantasy XIII games has been pretty epic.
While the secondary games have been really good, they've been lacking in their secret boss fights. The ultimate boss fights in the first couple of games came when Sephiroth (Final Fantasy VII) was involved. Links are posted below:
Julius (Kingdom Hearts 3D)
Xemnas (Secret Fight in KHI: Final Mix)
Anyway, those are my thoughts on what KHIII should be about. I want a more serious game that wraps up the plot--the Xehanort parts anyway--and expands on game play. A major complaint in the series has been a lack of ability to interact with the environment around Sora, and Kingdom Hearts: Dream, Drop, Distance provided a format to fix that issue. I think the creators have gotten themselves in a good spot, and I anticipate a highly well-received game.
Unlike a lot of the people on the internet that have overreacted to news that the title is "early" in development, I think the game will be out closer to the end of 2014 than the beginning of 2016. Have some optimism, people.