Just remeber that Se wanted a whole series on Final Fantasy XIII. Yes so far they have Agito and Versus XIII but they are not sequels to FF XIII. They are just side storys. FF XIII is its own big thing. As it sold so fast and the most in the franchise in a week im sure it will have a sequel. Se is already planning it btw.
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And NO again, Versus and Agito is not the same, same mythology different everything else. There's not going to be a Cocoon or Pulse about it. That's why XIII-2 was discussed about. Refining that story. 