I think I'm going to go with the oc being the soulmate of Faith and Angel, the oc will still be the twin sister of Buffy or the oc will be the niece of Joyce Summers and Hank but she grew up with the Summer family. The OC is the daughter of the Potter couple or I'm thinking FemHarry but the vampire slayer powers cancels out her witch powers. They'll be watching her life. Which idea do you guys like better?