I doubt Sazh will be playable or have a big role. His storyline was pretty self-contained. Snow either, though he is married to Serah so he must have some part to play.

You thought X was happy? You must have hated X-2 then. One of the worst parts of FFXIII was that it took itself too seriously. FF games need a level of whimsy. It's a game where characters have magical powers. They should have some fun with it. Gilgamesh and Ultros are some of the best characters of all time because they weren't taken too seriously.