I'd like to think that it's XIII-3, since the last words at the ending were "to be continued..." (I'm not revealing exactly WHAT the ending was, though, so no true story spoilers there.)

And I hope that Lightning gets an Aya Brea cosplay outfit like she did in Dissidia 012. Of course, I also think Serah would have looked cute in an Aya cosplay outfit, except without the rips in the jeans.