Nothing Is Finished by stuffandnonsense

Buffy Summers jumped from the tower to close Glory's portal in 2001, and Spike only survived her death because Dawn needed him. But Buffy's sacrifice didn't work. In 2023, the world is about to end and Faith starts travelling back in time, trying to fix it. Facing down her own death, Dawn has a plan to send Spike somewhere he won't have to lose everything: a dimension where Buffy is still alive.

The only problem is, so is her husband: Spike.

Like all of the Further Tales, this can be read as a stand-alone what-if. But you may appreciate it more after reading Ripples on a Hellmouth.