Alliance Raid storyline idea

At work today this idea for an alliance raid popped into my head, it's just a small idea but i can definitely work out details on my days off. Any Final Fantasy X fans in the house? Beware of spoilers if you've not yet played.

You know how Tural is it's own country you have to sail too? Well, what if there is a small island nearby or to the south of it...named Spira? From there is a boat that docks at Tural, or Sharlyan or Limsa, and off of this boat steps Yuna and Auron. Beasts are running rampant on Spira and they need our help, because something has happened. The Fayth are dreaming again...But this time, the dream has become a nightmare, and Auron has been brought back to aid Yuna in finding champions to stop the fayth. Something has gone very wrong, the story as we know it changed at a -very- crucial point. The twist idea I was thinking, is that instead of them defeating Yu Yevon, they failed to stop Braska's Final Aeon, and in order to save Spira, Tidus became Yuna's final aeon to stop it. So in order to stop the Fayth, the have to stop Tidus.

Like I said, just an idea and i can always fill out more details later, like locations and boss fights! Also, i have no idea how to spoil cover things on here, sorry T_T