I have a feeling there's gonna be some sort of plot twist. so apparently the 7 lights are Sora, Riku, Kairi, Mickey, Terra, Aqua, and Ven. Well I have a feeling that when Aqua leads them to Ven to wake him up, Master Xehanort will follow, stop them, and make Ven his 13th vessel. Most likely making Lea his replacement as one of the lights. I'm pretty sure Nomura has something like that in mind. Why else give Lea a keyblade?

But Lea is already one of the lights because Terra is a darkness. So if the loose Ven, they are one light short. Which means they have to bring Roxas/Xion in.