I'm still curious how it's going to be a prequel…if Nomura says it, it goes, but...it doesn't make sense.
Way to the Dawn, Riku's keyblade, is shown in the secret ending, but Way to the Dawn didn't come into existence until CoM. Also remember the words in the secret ending, "Memory of Xehanort." Xehanort didn't exist until a little bit before the events of the first game.
Perhaps a bit of time travel in this game? Or perhaps the events of the prequel are played along with the normal timeline? That would be kinda neat, do a level in the present, then a level in the past.
But I'm pretty sure that secret ending was intended to be in the future, even if the game itself is a prequel.