Sora may be ruled out actually. Do not forget the existence of Vanitas. Vanitas was essentially future Sora-Xehanort incarnate. We still do not know why Ventus' dark side looked like Sora, so there must be some sort of paradox involved. I would suspect it has something to do with Xehanort being able to be present in different periods of time at will. I have a feeling it'll likely be Riku, Lea, Kairi, Aqua, Roxas/Ventus, Xion, and an unknown seventh (maybe Mickey). Sora will become the thirteenth darkness, as foretold by Xehanort, but somehow in the end he'll break free of his bonds and become Sora again. Terra is currently a captive of Xehanort so no, he is not one of the seven. Ventus and Roxas are a toss up at this moment in my mind as is Xion but I have a feeling at least two of those three will be among them.
Mind you, I don't like the idea of Sora going bad, but the way Xehanort put it, it'll eventually happen. Plus, it explains the existence of Vanitas+Ventus. This all makes sense if you consider the fact that Xehanort has power to move through time (as is evidenced by his young form and the keyblade that he wields).