They're going to have to play very carefully with this. If they undone the actions of the previous games (for example, breaking up Tidus and Yuna, and bringing back Sin), they're going to have the whole "what the fans want" thing backfire on them, because that's not what most of the fans want at all.

Actually, though, I'm rather interested in the recurrence of Sin. Simply because I think it would shatter Spira once and for all. People have been living with the spiral for thousands of years, and then they finally move on and accept the Eternal Calm. And then Sin comes back again. Despair would completely and utterly consume Spira. It would have a tone more like that of Majora's Mask or FFVI's World of Ruin than Spira. It would be a world without even a possibility of hope. Everything that has been created, every tradition held on to, they'll just be gone. The world would have no future, nothing to look forward to. Some people would just give up, others would form weird cults. It would be extremely fun to watch and play through that Spira.

If we were ever to get it. But, unfortunately, we can be pretty sure that isn't going to be the case. Square just isn't up to that caliber of writing. They won't adjust Spira that much, (since it would be such a radical departure from the "fan demand") and we won't see that much impact on the people or their beliefs. Instead, we're going to get a bunch of nonsense story lines and pathetic character moments as they destroy everything that made the game and the world something people loved.