I personally like my idea for a X-3
Rebirth

it's not finished yet, but sufficed to say, Sin came back, using an unsent as it's core (Auron), Dream Zanarkand is reborn, tidus is pulled out of his life with Yuna on besaid, Besaid is then destroyed, killing Yuna, Lulu and Wakka. Yuna however stays as an unsent, and starts a campaign for destroying sin again.

Kimahri is later killed helping Yuna escape from sin on Gagazets base, finding a crucial weakness in the new sin. It has a poor defence and is much more easy to break through than the old Sin, possibly the result of using an unsent as it's core instead of a Final Aeon.

Anyway, With this knowledge, Yuna helps build a massive army from all the cultures on Spira, though the warrior monks needed bevelle to be destroyed to be convinced to help. Together they bring sin down, and Yuna manages to get to Sin's core and somehow disperses Auron, releasing Yu Yevon in the process. Using a bit of new tech for trapping loose pyreflies she manages to trap Yu Yevon in a sphere, thus preventing him from rebuilding Sin.

She also gets trapped inside it, as does Auron.

In the aftermath, Spira's leaders try to decide what to do with the sphere, destroying it would release Yuna and Auron, but also Yu Yevon and this would all start over again, so it is decided to keep it intact, and hidden. Kimahri appears at this point, an unsent like Yuna was, and takes the sphere to the farplane when he is finally sent.

Once in the farplane Kimahri finds the souls of Yuna's other guardians, Tidus, Lulu, Wakka, and Rikku who was killed a while ago in a freak accident, and they swear to guard Yuna once more.

yeah, I know it don't make much sense, but i'm enjoying killing the characters. Alas I'm only half way through the story right now and have been dying to finish it for months...