I believe sometime in BBS Aqua will meet Kairi. I think that she is responsible for Kairi going to the islands. She probably knew that Kairi was a PoH, and felt that if she sent her to the islands, she could protect her from Maleficent, because this has to be the time when Maleficent invades Radiant Garden. I think the order the stories will go are: Ven, Terra, Aqua. Because during Aqua's ending, I don't think Terra or Ven will be around anymore: Terra will be consumed by the darkness as Maleficent said, and Ven will be "erased". And something will happen to Aqua. But, since Nomura said that it doesn't matter which one you start out with first, I don't think we will know what happens with Ven and/or Terra if she is played first or second, but they will all tie into each other once all 3 are completed.