Buffy would still have to be an attractive blonde for me. I think that fits the stereotypical valley girl (so to speak) most, and I would keep it that way.
I think this thread is fun, but in all seriousness I hope it never happens. Why not give us something from the 'verse instead, the way Star Wars does? Don't remake the past, the past is already timeless; do a new show set in the same world and draw from the mythology; a new show idea would be best. It would be like redoing the original Star Wars movies to me: no point.