I always thought in the episode The Zeppo, they could have ran with it. What I mean is that, maybe the Gang could have initiated or somehow killed Xander and he could have conditioned the show as undead-Xander. While in search of looking for his 'thing' he actually gets something.. being dead, undead, zombie, etc.. My only problem with this is that it would have been overkill. The show needed to keep some regular people on there.. not everyone needs a special power or characteristic. A few seasons later of an Undead Xander, it probably would have gotten stale. Maybe have to write in a resurrection. Maybe not though.. the show probably could have used an undead guy on the team in a few creative ways.
Yet, I think that's the one I always think of.. anybody like or dislike or unsure of this idea.. for a filler expanding..