Buffy and Angel would have had sex, resulting in the return of Angelus. Then either Buffy would have had to dust him, he would have had to kill her, or there would have somehow been some way of restoring Angel's soul.
Assuming that Buffy were turned, then it were to be discovered that she was pregnant, pretty much the same thing that happened with Darla, where Connor's soul influenced his mother, would have happened to Buffy, but Willow and Giles would have found a way to restore her own soul and possibly her humanity along with it. (Angel would, of course be totally oblivious of all of that, because he would be back in Los Angeles helping the helpless) The Watchers' Council would take the baby, either with or without Buffy's permission, resulting in a Buffy/Giles feud similar to the actual Angel/Wes one) some twist of fate would have caused him to age 15 years in 3 months, and Connor would have shown up in Sunnydale, immediately had an affair with Dawn, and she would become pregnant with what would become Jasmine, and that whole story line would have played out in Sunnydale rather than Los Angeles, with Angel and his team just as oblivious, or at least unconcerned, as Buffy and the Scoobies were in Joss' original story line.
Once Jasmine was defeated, Dawn would take the nearly insane Connor, using her powers as the key, to the reality without shrimp, and neither one would ever be heard from again.