Pretty sure there was a story like this at some point but it got taken down.

Before Buffy jumped to her death in “The Gift”, she slept with Spike. When Willow resurrects her, she essentially re-animates Buffy’s dead cells – including Spike’s sperm, traces of which are still in her body.

In other words, not only does the spell resurrect her, but it also gets her pregnant.

She has to find out before “Once More, with Feeling” (if that episode even happens) but no sooner than two weeks after the conception/resurrection.

So instead of the sexcapades with Spike, Buffy is dealing with being pregnant while Spike doing everything he can for her. Up to the author whether or not he gets a soul.

When Willow finds out, she gets an ego boost from the knowledge that she created life from a vampire. Optional whether or not two lives are taken from Willow as payment instead of one (so Tara and someone else she cares about, like Xander).

Bonus points for Buffy yelling at Riley when he comes back.

If the story goes as far as season seven, then Spike can’t die.