This thread contains massive spoilers for both FFX and FFX-2.

Okay, when I first beat FFX-2 perfectly and got Tidus back, I was in complete disbelief that this could happen in the world which FFX left. I thought to myself that if Tidus could come back, then so could Sin and all the Aeons, thus totally nullifying all which had taken place in FFX. The more I thought about it, the more it came to make sense that there was a logical way for Tidus to come back without throwing away the facts pressented in FFX. First we have to start with the relation that everything has to everything - ie: Sin.

What is Sin?
Sin is the armor Yu Yevon wears to bring its chaos. SIn itself is the mutated form of the Final Aeon Summoned at the final battle between Sin and a Summoner. The Final Aeon is not an Aeon of the Fayth, but an Aeon created out of devotion to a Summoner. It is another person sacrificed out of love/friendship.

What is Yu Yevon?
Yevon was the govoner of Zanarkand 1000 years before FFX took place. He was Yunaleska's father. He was also a Summoner, as was Yunaleska. (This is why I think they named Yuna after Yunaleska because she too followed in her father's footsteps as a Summoner). Zanarkand was the city of summoners back when Yevon ruled it (so it is said). Yevon would have rather destroyed Zanarkand himself then let Bevelle destroy it, so he did. He then turned the populace of Zanarkand into Fayth. He used his Aeons to create the armor that is Sin. The only method that could defeat Sin was the Summoning the Final Aeons.

Why was Sin there?
The games tells you it is punishment for Spira's "sins". This isn't true. By taking control of the Fayth's Final Aeon, Yu Yevon forces Zanarkand to be recreated in a dream the way it was when he destroyed. Hence why, at the end of FF you see Jecht in what appears to be Zanarkand. When the Final Summoning occurs, this dream of Zanarkand is destroyed, which is why Yu Yevon in turn takes control of the Final Aeon so he will be reborn again, and continue his dream of Zanarkand.

Why do the Fayth need Sin to be destroyed in order to sleep?
By now this should be obvious. Yu Yevon is forcing the Fayth to dream his dream of Zanarkand each time he takes hold of another Aeon. Once all the Aeons are gone, Yu Yevon will have no hold over the Fayth, and therefore, the Fayth can sleep.

What does this have to do with Tidus?
Tidus is a Dream of the Fayth; he is a being of the dream that Yu Yevon is forcing the Fayth to dream (Zanarkand). Because they wanted to rest, he dissapeared, leaving Spira and Yuna at the end of FFX.

How did Tidus get to SPira from the "dream"?
Sin can't control this dream since it is forcing the Fayth to dream it. My interpretation is that the Fayth were waiting until they could find the dream that would end all of their dreams. It started with Jecht and ended with Tidus. Jecht used Auron, an unsent, to go between Spira and the dream.

If the Fayth sleep, how come Aeons came back in FFX-2?
I believe this lies in the connection Vegnegun had with the Fayth and the Pyreflies reaction to Yuna's memories. The Fayth can't sleep as long as Vegengun is awake. Therefore, a butchered reincarnation of the Aeons is possible through the connection Vegnegun has with the Aeon's Fayth, and the connection Yuna's memories have with the Pyreflies. Yuna and friends are the only ones to see the Aeons, so this theory is supported by the Farplane facts. Other's memories are visable to others while they are there. The memories made real by the Fayth who are now awake, and controled by Vegnegun who wont let them sleep. That's my take on it anywho =3

So... Tidus can come back... how?
The Fayth are mearly sleeping, they are not gone nor dead. Tidus is their reward to Yuna for saving their butts, AGAIN (at least I think). Like Tidus said, the Fayth gathered up his thoughts to recreate him. He exists again becuase of the bond Yuna and him share. As long as this bond is never broken, Tidus will remain by her side... or so he thinks (Tidus isn't too good with for sures).

So, since Yu Yevon is gone, Sin can never return. And even though the Fayth sleep, Tidus's memories of himself and thoughts create him, as well as Yuna's bond to him. Stranger things in FF's have happened, so I guess this I can buy. At least Yuna had to work REALLY hard to get her man back XD;

But yeah, that's just my take on the whole thing. You don't have to aggree with me, but after lots and lots of thought, that's how I think it could work out after examining all the facts from FFX. It's the only plausable explanation, ^^;

I'll stop blabbering now with my insane FFX fandom x_X;;