The thing is, though, that Halloween fics so saturate the Buffy fandom, that throwing something slightly more contemporary into the mix can be the only way of making them even slightly more interesting.
It doesn't help that I'd sell the important body parts of other people to see EMIYA! or Gilgamesh! Xander. And, as it happens, one could quite easily twist the episode to accomodate the latter.
Let's say his History or English or Religion class are looking at the Enuma Elish, and Xander decides to dress up as the badass God-King. Have Ethan's little spell replace Xander with the closest metaphysical equivalent of Gil, which would here be borrowed temporarily from the Throne of Heroes. Voila. Extra points if you can swing him keeping the Key to the Gate of Ownage. With the shit Gilgamesh collected over the years, you could throw all sorts of elixirs and such to make Xander stronger or give him more Prana, but still stick to BtVS power tiers.
Perhaps, though, I'm not the best author to comment - I once started a fic with the A/N; 'Computer technology is ahead by about a decade here' so that I could have Xander dress as Vergil. I feel no shame, though - it was a hell of a lot of fun. ^_^