Instead of dressing to please Angel on Halloween, Buffy dresses up as a vampire. While Ethan's spell is in progress Buffy sleeps with Spike. When the spell is broken, Buffy comes back to life. About a month later Buffy discovers that she's pregnant.

Could Have:

Angel could still lose his soul, just not by Buffy

Can't Have:

Too much Angst

Buffy/Angel sex