There are plenty of stories that mix Stargate and BtVS. Usually it's in one universe. Sometimes someone or multiple someones cross universes but always their abilities stay the same.

What if SG-1 or SGA-1 crossed over to Buffyverse and some of them acquired magic talents or slayer abilities? What if the Scoobies (or just one of them) ended up in the SGverse where suddenly Willow's witchiness or Buffy's Slayer essence are no longer. Something has to account for Slayer abilities and so the demonic essence inside the slayer takes shape and becomes an alien entity inside the former slaye...

My initial idea was for one of the ladies of BtvS to wake up in Anita Blake land as Raina (the bad guy in early ABVH books). And then go from there. Confusion, mistrust from others, trying to make things right - be a proper lupa, etc. Any point in the ABVH timeline - she could even wake up from the dead (as Buffy did-kind of).

But on second though any fandom will do. For example Stargate: a goa'uld or Adria, Star Wars: Vader, Revan (pre mind wipe), Malak, Darth Maul, etc. The key element is the snapping out of it / waking up bit where the BtVS character starts out.

You can ha...

Crossover with the movie Troy.

Buffy, Faith, or any other slayer (Kendra, Vi, etc. thought preferably not Kennedy) wakes up to find herself in Ancient Greece. Furthermore turns out to be Andromache, Briseis or Helen. They wont stay passive for long.

Either the slayer looks the same as she used to but everyone around insists she is Andromache/Briseis/ Helen, or her body is changed the way it looked in the movie Troy (for example Buffy becomes Andromache and suddenly is tall and brown haired and has to get used to it).

Depending on the time you chose, the slayer will ...