So apparently I am cheerfully supplying many answers. One per story series. I just wrote an origin of Slayers story for this new Greek-myth-influenced series, and it was a lot of fun to do.
I've got a totally different origin story for the "Breaking Glass" universe stories. And another one for "Cloud Animals". A magic system in "Breathing". Another magic system in "Reconnection".
I think I have the most fun when I stop worrying about whether things are good in some abstract sense, and just worry about whether they're good in the "I'd think this was cool to read" sense. When I write for my future amnesiac self, I do best at pleasing all of you as well as me. And my inner amnesiac wants neat backstories.
My inner amnesiac wants to write "A rose for Ecclesiastes." It also wants to write Against a dark background, but probably with a less horrifically grim ending. (That's a good review, by the way, and I strongly recommend Banks if you have any interest in SF. Best living writer of it, to my taste.)
What do you want to have written?