We're going to play a little game of pretend. Yes, this takes place in Spira, and yes there are summoners and there is the evil that is Sin. All of the locations are the same, as are the names of the people. One or two people will be of my imagination, but other than that, it will be the Spira of FF X.
Here are the key things that need your imagination.
1. Yevon is not evil. Sure, there are bad people in charge at one time or another, but overall, Yevon is not the bad guy. They are what help the summoners keep Spira going. Sort of the muscles that keep the backbone supported, with summoners being the backbone. Seymour is still creepy Seymour, but not hell bent on destroying Spira. Mika, that old coot, is dead and buried (praise be to Yevon!) and the temples are still a key part of the world.
2. Sin is a little different. It doesn't just cause damage or kill people. What it does, is suck the life force out of all living creatures and plants. That's why no one can truly kill Sin. As long as there is life on Spira, there will be Sin. The Final Summoning will temporarily trap it, but it will always escape because there is nothing that will hold it for long. Its evil is like acid, eating away at the bonds that keep it from Spira.
It's also not the person who was chosen to be the Final Aeon. It is Sin. There is nothing recognizable as 'human' in Sin. No being calmed by the Hymn of the Fayth, no trying to communicate with anyone. There is no 'help' from Sin by bringing someone from another world/dream into Spira. It just sucks out the life like the marrow from bones. Got it? Ok.
3. Yunalesca is still cold-hearted 'let-me-save-you-by-killing-you' Yunalesca. That whole 'hope' spiel is a bunch of crap. I hate her, but she is necessary. meh.
4. Summoners still go to temples to get Aeons. They perform sendings. But the most important thing they do is to bring life back to Spira. They restore what Sin takes away. This means that there are a lot of them, and not all of them have the strength to go against Sin. Very few have what it takes, and even then they go in pairs with their Guardians to Zanarkand with the Bitch-woman to defeat Sin.
The main point of pilgrimages is to bring back the life force of the world. Every point in the pilgrimage is where they perform this feat, called 'Restoration'. It means that the crops will grow, children will grow and adults will stay healthy enough to care for both. It's also performed where ever there is a need of it. Summoners heal the wounded and dying.
5. Jecht and Tidus will be there. Not from the city of Zanarkand, but from Spira. Jecht will be dead of course and Tidus will be on Besaid. Got to have some Tuna-ness going on, right? lol.
6. The Al Bhed aren't exactly hated, but they aren't exactly a part of the global community. Machina are not widely used by anyone BUT the Al Bhed. They are the ones who put it to use for the good of mankind. No one else is willing to do it, cause of the whole taboo on machina from Yevon. Yevon still says what machina can be used and what can't be used. The Al Bhed don't take part in Yevon beliefs.
Hmmm...I think that is about it for the important pretend issues we needed to hit. Make sure you read them carefully, cause while its still Spira; it's not the Spira you're used to. Let us get to the good, juicy bits! On to the story!