Well, Now that Kingdom Hearts 3 has been anounced. I think that a good speculation is in order. This is what i believe will happen/will have in KH3:
Lets start with the obvious.
Riku will free Aqua and Ansem the Wise from the Realm of Darkness, but Sora is the only one who can free Ventus since Sora carries his heart.
Once Ventus is freed, Roxas will no longer look or speak like Ventus. He will now look and speak like Sora.
You won't fight Xehanort. You'll instead fight Terra-Xehanort. That's how Terra will be freed. Maybe you can fight Xehort's ghost or something as a bonus.
Now my personal predictions.
There will probably be a Jungle Book world.
Sora and Kairi will kiss. I mean...cmon. They gotta by now.
A new love interest for Riku? Yes. It's not Xion or anyine familiar. It will be a new original character...one who's personality will be nice and friendly (maybe like Shiki's from TWEWY but NOT Shiki) and can bring out the "light" in Riku's heart.
Yen Sid will fight! Imagine, at the final battle...when all hope is lost Yen Sid flashes in through a portal holding a badass keyblade. The former master will now take arms and fight alongside the heroes.
New powers for Kairi. After all, she is still a Princess of Light. Maybe she will have awesome light based combos with her keyblade...which lead to...
More playable charcters. I'm sure during the story of KH3 you will get to play as Sora, Kairi, and Riku.
What do you guys think? What are some theories of your own?