There's Light Blue by Spuffy93 part of the series Shades of Blue
Summary: Spike, Buffy, and Harmony are talking outside the fraternity house when a strange demon appears and attacks everyone without distinction. Harmony escapes as Spike and Buffy stay to fight the demon together. They win, but then a bright blue light engulfs them. Things as they know it will change forever.Best Wishes
Summary: Beginning just after the events of «The Girl in Question» a benevolent vengeance demon grants an inadvertent wish made by Buffy in Rome that sends Buffy, Spike, and Xander back to the middle of «The Harsh Light of Day» where they are given a chance to change their past lives for the better.Hope
by Wonder and Ashes
Summary: Spike becomes Buffy's unlikely protector when she discovers she's pregnant with a mystical energy known only as the Key.
I'm gonna look for more, but in the meantime you can find some recs here (if you didn't search in the forum already) :Buffy and Spike are Dawn's Parents.Dawn as Spike and Buffy's daughter
Didn't see at first that it was you who posted this one.)
It's slightly different than what you're looking for, but I know Dawn, after sacrificing herself, is reborn as Spike & Buffy's in some stories if you're interested.