First off, if a new "Buffy" movie were to be made, whether or not Joss would be involved in any way, I wouldn't want any of the original cast involved. If, however, we absolutely had to stay with the original cast, and Nicholas for some reason either could not, or would not, reprise his role, I would say that there should be a simple statement near the beginning of the movie about Xander being in the jungles of South America, or some other such place, with a team of Slayers, tracking down rumors of some kind of demon infestation.
If the movie is supposed to be made with all the real (now middle aged) cast, bringing in a ringer would be cheating.