OK, I posted this musings in a more organized form at Fan Fiction.net, with my first crack at possible stories. I will keep on posting new stuff there as some of the "short stories" are getting pretty big
Check it out:

1: Authors Note: How it fits FFX and X2
2: Prologue
3: 1,000 years ago: Sin and the Zanarkand war
4: Yu Yevon and Bevelle's Betrayal
5: Yunalesca gives her life for Spira
6: Spira's Spiral of Death Ends ?
7: Appendix Spira's Ancient History (5,000 BT thru 3,000 BT)

http://www.fanfiction.net/s/2685724/1/

BT = Before Tidus