Theory: I believe the game will be a prequel or if in the future will dive into the past a lot. Nomura has answered KH1's questions. He left a bunch of questions on the keyblades IMO. I believe that he needs to explain to us about the keyblade to continue into the future. Right now, the future could be ANYTHING. The past seems more defined. There's not enough information to tell you what the Chasers are. My theory is they are enemies. They don't exactly have the nicest sounding name. Xehanort's Memory might've been what we saw. It could also be about the keyblade. He had no memory about his life before Ansem the Wise really. This couldn't be just another thing to make him more mysterious. Birth by sleep, I believe this refers to the keyblade. It was born through death. I also believe that if it is in the future ,as many believe, that it could just be referring to how Sora was born as the keyblade weilder through a dream while he was asleep. I don't think they'd emphasize it that much though if that was it. It won't be a numbered entry so this causes me to think it will be a prequel. Also, I believe the four people are each a different keyblade master. Twilight, Darkness, Light, and Present(?). This is the basic idea behind my theory.
What Worlds Would I Like To See?:
Narnia
Emperor's New Groove
More Tim Burton Worlds(He's my hero. They did Nightmare Before Christmas.)