Ok Angelus is a vampire overlord kind of like he used to be only difference is he was never under Darla's thumb he was one of the ancients the original ruling class of vampires.

They even have their own country just the humans don't realize it exists maybe because of a parallel dimension thing. Anyway he hears about the Elfin Princess who is of renowned beauty, he decides he wants her for himself and threatens the elfin people unless she is given to him, her Father and brothers refuse obviously, but she finds out and offers herself to him, and here's the thing: she's only half elf so at one time she lived in the world of mortals and saved Angelus' life, and she has loved him since childhood, anyway when he realizes who she is he offers to marry her and make her his mate instead.

So they get married everybody is happy for awhile until he finds out that she is pregnant. He is obviously angry because vampires can't have kids, he asks her why she cheated on him and who it was, he gets angry and fights with her repeatedly, he hits her and stuff like that, he has an affair with Darla and they head off to Romania where he is cursed with his soul.

Of course he can't go back home so everything proceeds as it has except excluding Buffy and he ends up in L.A. with his business and one day who should show up but his wife. Kind of an awkward situation, she's not the sweet little girl she used to be though. She has her own little gang of friends. She learned to be tough the hard way, and is mistrustful of him. She also single handedly held together his entire empire for 200 years.

This leads to all sorts of interesting situations. They eventually get back together and eventually he finds out that vampires can have children with elves, and of course the inevitable question 'Why didn't you tell me?'

'You never asked.'

Notes/Semi Requirements:

Angelus really loving the young Buffy, having a gentle and loving relationship, that's why he goes into a jealous rage when he thinks she has betrayed him.

Young Buffy is the kind of sweet innocent sort, just to make the transformation an extreme.

When Angel bumps into Buffy again, he and the gang were going to take out a bunch of vamps which were having a war/battle. They watch to see which group will be the winners so they can take them out, and are surprised by Buffy's fighting skills. She is dressed in black leather (Angelus-y type dress) Angel's shocked when he realizes who it is. When he starts talking to Buffy instead of trying to kill her, the gang is like, you know her? 

Angel, 'She's my wife.'