However, on reflection, I would be *all over* an AU in which Ruby was some *other* kind of formerly human possession spirit (they abound)from another religion and/or culture.

Everything in present day would be the same minus Tami and search and replace on demon and hell in her meager backstory right up until the whole "I DID IT ALL FOR The Money LUCIFER! I WIN!". She could *totally* be working the whole "Use your powers, screw their moral scruples about your powers, screw their apocalypse" from the motivation of "Your Judeo-Christian Apocalypse can TOTALLY BITE ME".