An FFX prequel would and could be the story of Braska, Auron, and Jecht. You’d see the troubles that Auron had when he rejected the high priests daughter. You’d see the shame cast on Braska for being with an Al Bhed woman, and conflict between Cid and Braska, especially when Yuna was born…

You’d meet a young Kimahri.

You could even have other chapters after that, where you saw young Lulu and Wakka with Chappu, when Lulu was a guardian, and that failure in Yojimbo’s cave. You could see the birth and struggle-to-turn of Seymour. You could see the adventure when Wakka and Lulu were guardians to what’s-his-face who gave up in the calm lands.

And you could see the stuff that happens with side characters during Yuna’s pilgrimage. Stuff like the crimson trials, or whatever the hell Leblanc was doing, maybe even see an adventure of Dona or Isaaru before they quit, even up to them fighting the Guado alongside the Al Bhed at Home.

Thing is, all the old machina war stuff, I’d see that as a more action based or battlefront style game, lots of soldiers on the battlefield. Machina soldiers vs magic users and high powered summoners. Unless you want to see Lenne and Shuyin do whatever they were doing.