It depends whether or not it came off the ending where Tidus returns or if he didn't.
If he did, Tidus, Yuna, Rikku, and someone new, possibly a romance for Rikku?
If not, Yuna, Rikku, Wakka and again someone new.
They'd be looking for Tidus and like find a way through something to him, and end up in a weird world and half the game would be played there looking for Tidus and Jecht and Auron (or did he actually die?)