It would be my belief that Fenrir (along with Belial and Bahamut) would probably end up in 'The Great Labyrinth of Bahamut' - the 24-man RAID that is supposed to be AFTER The Binding Coil. Either that or Fenrir could end up on one of the next floors of The Crystal Tower.

This being said, I would absolutely love the idea of them adding him as a Mount Drop like the Nightmare.