Would it happen? Heck no. There are so many interesting ways to tie in another game properly. It would need to be a character tied well to the story, yet someone who's story isn't easily assumed. A character like Weigraf (involving the 50 year war, leading up to his ascention to White Knight, and inevitable death) or Vormav (what lead him to do all that he did, his motivations and manipulations before he is introduced). As mentioned before, we'll never see something like that. Square just doesn't have it in them.