Oddly enough, since somebody mentioned Highlander and Xena, the one crossover I *really* would have loved to see on screen is Highlander/Buffy...
By Jove Magpie - I do believe you've found one I could live with !
It just seems logical that the Immortal could so easily be assumed by the Scoobies to be a vampire....
And we saw how well vampire-killing techniques work on Immortals in the series (i.e. not very - Immortal pulls stake out of chest, and exclaims "that bloody *hurt*!")