I've been wondering this ever since I started replaying the game.
Dream Zanarkand is supposed to actually exist in real Spira, somewhere far out at sea. If that's the case, then anyone should be able to go there, right?
Why not make a game about that? Like a group of people setting out to find it during the events of FFX, find it and try to keep it there. Like trying to stop it from going away? I don't know, but it'd be interesting if there'd be a player in the story that wants to keep Dream Zanarkand. Like a Fayth that wants to keep dreaming.