Evil Set-Up: Faith, Angelus (on the Hellmouth) and a blue minion of Evil's choice. If Kathy, she should start on the Hellmouth and Angelus somewhere else, maybe on the Master's start space. I don't really care, unless there's a Mansion on the board.
Artifacts: 3 non-story artifacts, and a story artifact, which will be the Soul Switcher.
Evil Objective: Destroy or sire Buffy the Vampire Slayer, unless this turns out to be too easy, in which case kill them all.
Good Objective: to have Faith and Angelus both be out of play. I say it this way because Angelus can be resummoned (imagine Master-style resurrection, or some time spell; it's more realistic than messing with the Moon) so simply having killed him doesn't count. Faith and Angelus must both be dead (or Good Angel) at the same time.
Special Rules: No black die; the three "minions" always move, unless sired characters break in. Vampire Faith (Faith can choose to be sired by an evil vampire, resetting her lifepoints.) Evil Magik: spells are cast by a particular evil character, adding significance to the choice of Darla (more life, less Magik) vs. Vamp X, and adding pathos to Faith if she's all alone. Also, note that while Faith and Angelus are the surrogate Big Bad, their artifacts are still face down until used. I wouldn't want to lose the element of surprise.
Soul Switcher: this artifact is initially useless to anyone besides Faith. If Good gets it they should destroy it. Faith uses it by attacking Buffy and declaring that she's using the artifact, in which case she doesn't roll dice, but switches bodies with Buffy. The Buffy token becomes Faith-in-Buffy, an Evil character who moves during Evil's turn. She draws black cards for Evil, but she also keeps her Good cards, and can draw and use more. This reflects Faith stealing Buffy's life. The Soul Switcher artifact is discarded (see below.)
The Faith token becomes Buffy-in-Faith, a Good character, controlled by Buffy's player and moving on Buffy's turn. (Which should let her run away.) She can't draw Evil cards (and wouldn't want to) but can't draw Good cards either (her life's been stolen, no one besides the Good characters will believe or help her). If this turns out to be too harsh, she can hold onto one card at a time. But I think it's important that she not be able to act normally; Good needs a big incentive to try to restore Buffy to her body, not just kill Faith-in-Buffy and replace her with Buffy-in-Faith.
Note that the victory conditions apply to the soul (or in the case of Vampire Faith, controlling personality). If Buffy-in-Faith dies or is sired, Evil wins; if Faith-in-Buffy dies, Buffy is pissed off, but Good has become closer to winning (or wins, if Angelus is dead or Angel.)
No life points are altered in the exchange; we're changing spirit, not flesh. Faith-in-Buffy has the life points Buffy had, and vice versa. Buffy-in-Faith can be healed by Spells of Healing if keeping Buffy alive outweighs the drawback of fixing Faith's body for her.
Undoing the change: There are two ways to go. To imitate the relevant episodes, the artifact itself is gone. But the Spell of Soul Restoration card can be used (at the option of Good) to create a replacement artifact (like Willow did, giving Buffy-in-Faith a glowing thing to go after her body with) which Buffy can use just like Faith did. Given the difficulty of finding the spell, success should be a single magic splat, and perhaps can be cast anywhere; after all Buffy still has to then catch up to Faith. Once all that's over, Faith is stuck in her body. For game balance Faith-in-Buffy might be required to discard that Spell if she draws it (since there's only one), but maybe not.
The other way is to discard the artifact to a random corner, and then everyone can go race for it. (Note that Faith-in-Buffy could hold it, or destroy it with the Living Flame.) Going this route, one could also make the artifact a general device for switching human souls, not just Faith and Buffy, allowing Faith to jump from body to body. If you do do this, Magik should follow the soul and fight dice the body, so that Faith-in-Willow would be a 1/1, and Willow-in-Faith or -in-Buffy would be a 3/3. Obviously Faith would have to be desperate to do that, but she *might* like Oz's body.
Faith-in-Buffy still has the option to become a vampire, but this doesn't count as "siring Buffy" if Buffy-in-Faith is alive and kicking, and it has drawbacks: as an Evil Vampire Faith would lose her Good cards, just like sired characters. Also, the Soul Switcher only moves human souls; if Buffy-in-Faith hits Vamp Faith-in-Buffy with it, Buffy's soul moves back to her body, making a Good Vampire Buffy, but there's nothing to put in Faith's body, and she dies instantly.