Near the end of FFX, I went to Mt. Gagazet and spoke with Maechen. There's a few things that made me think of what was going on there.
Bevelle and Zanarkand were having a huge war 1000 years prior to FFX. Near the end of that war, Bevelle's soldiers were going through Mt. Gagazet, and their plan was to attack Zanarkand. However, they saw some huge creature on their way, which I assume was Sin. The soldiers ran away in terror. Another group of Bevelle's soldiers went through Mt. Gagazet later and saw Zanarkand completely destroyed. That was done by Sin too, I assume. However, Sin was the summoning of Yu Yevon, and Yu Yevon was the leader of Zanarkand.
Why did Yevon turn against his own city and use Sin to completely destroy it? And what does that have to do with the fact that Bevelle banned the use of all Machina a while after that? As far as we know from FFX-2, Bevelle was developing Vegnagun to destroy Zanarkand. That kind of weapon could have posed a challenge to Sin as well. However, Vegnagun was uncontrollable and so it was never used. Fearing the power of such machines, Bevelle could have banned Machina as a result. However, that doesn't explain why Bevelle still kept using Machina while denying others the use of Machina.
Finally, what was the point of giving Yevon all those Aeons and then killing them? You can let him take over 5 Aeons (Valefor, Ifrit, Ixion, Shiva, Bahamut) and kill them if that's all you have, and that does it. However, if you have extra Aeons, you still need to sacrifice them too. Does that weaken Yu Yevon somehow? If Yu Yevon was a powerful summoner, Yuna could have just blown him to oblivion with her Aeons and everyone's attack powers, and if Yu Yevon was an unsent, Yuna could have Sent him.
And now that I think about it, why didn't Jecht just stab himself, jump down from the Zanarkand platform, kill himself or let the party kill you? Or even better, I think a good plan would have been to use Jecht against Yu Yevon, Star Wars spoilers(Darth Vader killing Emperor Palpatine comes to my mind) seeing how Jecht was the Final Aeon. So even though Tidus, Yuna and the others got Sin and Yevon defeated and brought an Eternal Calm, I still think their plan was rushed, and they didn't completely think about what to do. To quote Yuna from FFX-2, "I don't like your plan. It sucks." Fortunately Yuna realised not to repeat the past mistakes in the final battle of FFX-2.
During the ending sequence, Tidus, Auron, Jecht and the Fayth went into the Farplane. Doesn't that mean Yuna and others could possibly revisit them if they go to Guadosalam and/or the Farplane? I also remember seeing Bahamut's fayth (the child) and hearing Auron/Braska/Jecht's voices when you're in the Farplane Abyss in FFX-2.
Okay, there were a lot of questions from both FFX and FFX-2. I wasn't sure which forum to post this on, but since the questions were more about FFX, I thought FFX forums would be a better place for them.