If FFX's last boss was actually Sin itself... that'd be pretty incredible - the colossal floating demon, faced head-on in a fight to the death, rather than the little skirmishes where it's faced earlier. Or better yet, attacking it from the inside, bringing it down that way. Since (SPOILER)Yu Yevon is the essence that summons Sin, he could've bonded himself to Sin's body, so the fight would still be consistent with the game's plot. However, the FF creators know what they're doing, and I'm happy with the way things turned out.