Ok this is a pretty easy bunnie, first pick how old is Xander. Is he on his post-grad trip, after Sunnydaale fell in, returning from Africa or some other time. Then pick the universe he's in. CSI Miami, CSI New York, M7 or something else.

Then have them find Xander who is suffering from amnesia. They take an interest in Xander and trying to help him find out who he is but a problem arise as people (Order of Taraka) keep trying to kill him.

Figure Xander got amnesia from either a head injury, too much stress and the brain rebooted or Willow deciding that Xander should no longer help.

I can see Xander helping hunt down people in Miami or with M7 using his hyeana sense. Or see him with Gibbs, soldier meet soldier.