YAHF
We all know that most of the time Xander is powerless, so what happen when during a portal night before Halloween he "found" some money that was drop? He use it to get a costume of someone or something that is the opposite of powerless and keep it.

-Xander centered
-He went as someone or thing that is near omnipotence such as Doctor Manhattan (Watchmen), Onslaught (Marvel), or The Beast (Infamous) to name a few that maybe use
-Can be a mult cross over if you wanted to be
-pairings are up to you
-NO one shoot!!! This is the major point!

Xander was know as a geek, so why not go with something geeky for Halloween?

-Xander dress up as mage for Halloween.
-I most like if you use the game "Magicka" as the base for his magic.

One Vision, One Purpose

Buffy/Stargate/Command&Conquer - YAHF

Xander was know to be the self-proclaim king of the nerds, so why not dress up as one that only nerds know of? In real if he is a unknown hero, why not go as something opposite? Combining those two thoughts and add some black and red clothing, you have one of the most iconic villain in RTS history: Kane.

-Xander dress up as a modified Kane during Halloween, like have head hair. Please have Xander keep Kane's awesome beard.

-I'm thinking that adding Oz as Einstein. Oz is a smart guy, but he jus...

Xander and Oz was both persuade to help Jonathan and Andrew get enough money to rent a set of costume from Ethan's shop on that faithful Halloween night. Good news is that there are four more superhuman soldiers out in there, bad news is that there are four more superhuman soldiers out in there. Can those that goes bump in the night survive what is that is to come?

-The four guys dress up as 4 Ultramarine space marines.
-Xander is the leader and was dress as Captain Cato Sicarius. Oz dress as a Techmarine so can use his smarts to built and forge equipments for the team. Jonathan ...

A YAHF with "The Elder Scrolls 5: Skyrim"

Xander got some more money to buy a costume at Ethan's shop. So he rented a viking costume that was more then meets the eye and now dragon shouts can be heard in Sunnydale.

-Xander dress up as a Nord dragon-born like the one that is on the trailers and aids.
-The character is good weapons and shouts. Him being able to do others things like magic and forging are up to you.
-Pairings is up to you.