I think Aqua's story will be more important than Terra's and Ven's. We still don't know what her story really is about. Terra's is about him struggling with his darkness and Ven's is about him wanting to understand his place in the world. We know Terra and Ven's personal issues, but not Aqua's. The only thing we know about her story, is that she's trying to protect Terra and Ven, but we haven't really seen anything personal about her. Her personality is so much "brighter" than the other two. I think she acts like this to hide something, something deep. Her personal struggles might be more complicated than Terra or Ventus, and that's prolly why Nomura said that story wise, her scenario should be played last.