My personal opinion is that Nomura is creating a spin-off series set in the past to explain the history of the Keyblade, and that the spin-off series (probably will consist of two or three games) won't be titled "Kingdom Hearts: <insert title here>," rather something different but similar. I also believe it will revolve around the three "enigmatic soldiers" and so on in the secret movies. I also believe the spin-off series will elude to Sora, but won't actually talk about him because the series is set in the past and Sora either doesn't exist yet in the time period, or it is set before Sora obtains the Keyblade.
My opinion also is that Sora will remain as the main character for the actual base storyline, and will return for the next Kingdom Hearts game with Kingdom Hearts in the title. Nomura has said that Sora's story is not complete, so he has... in a matter of speaking... promised at least one more title with Sora as our hero. That alone makes me happy because I highly doubt that by now Sora is perfectly content with staying on Destiny Islands. I call it Linkitis. After the events of Legend of Zelda: Ocarina of Time, Link was not content with just submitting himself to a child's life once again, and so he sets out on a follow-up adventure in Legend of Zelda: Majora's Mask. I believe something new will come to light (pun? Bad one, not intended) in the next game starring Sora and he'll have to vanquish whatever it is. Whether it be a new enemy, or a familiar one, I have a feeling Sora's story isn't going to end right back where Sora, Riku, and Kairi started.