This could be the best day of my life. On top of everything else, my gift was posted at sirry_slash and it's wonderful. Harry goes back in time... which may be cliche, I don't know, but it's what I asked for, and I got the most beautiful wonderful Sirius/Harry I've ever read. It's compact. It's economical and expressive at the same time. Oh, and it fits me like a glove! My Secret Sirry Santa did a fabulous job!!
It's called This Time, NC-17. Short enough to read RIGHT NOW.
p.s. I would also like to mention that my original prompt wasn't nearly that long. I was asked for more to go on :).