Could, but I would prefer it wouldn't. At first I thought I would hate the idea of the two together, but I'm not as sure anymore. Xander worships Buffy, but Buffy does not return the sentiment; this is not good relationship-building material. However, Xander is as devoted as a friend could be. Sometimes a little too devoted, but he can get away with it cause he's so cute and charming. Being a smartass does have its advantages sometimes!
Although these two wouldn't make the perfect couple, the chemistry is there. I'm sure all of you remember the episode When She Was Bad, and the infamous Bronze scene in which Buffy draped herself (and other portions of her anatomy) all over Xander in front of everyone. That scene was by far the best in the show - the lighting kicked ass, the music was perfect, and the other stuff wasn't so bad either. But it also revealed how great the sexual tension really is between these two friends.