I remembered the other title but it was never completed and from your description it probably is NIAMH' s story you were thinking of. It's a great story.
But since I remembered the other title I'll post it here. The other story is on this site:
The Mysterious Youth ---by Karin-----wip
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=16113Summary: Season 5 – Post Out of my mind. Buffy and Spike are living their lives, disliking each other. But when Buffy gets a vision, and makes a, what seems to be a harmless Christmas wish, a girl appears into their lives, telling them she is from the future, years after Angel's final battle. What happens when this girl claims to be their future daughter, and tries to prevent the mistakes that caused her parents to die?