Other popular themes at Halloween are often pop-cultural references, with costumes based on characters from comic books, movies, TV shows, board games, fast food restaurants (a la
Burger King)... you name it. Some of these have been featured in games - and some of those games are even multiplayer party games. There are laughs to be had with the
King Games for the Xbox, of course, but don't forget games such as
Marvel Ultimate Alliance, which could be awesome at a Superhero - themed costume party.
Another thing to consider is that the above scenario works just as well the other way around. Videogames are pop-culture, so you can chose a game or variety of games that are to be played during the party and have party goers put together costumes to match the theme. For a game that has customizable characters, such as Karaoke Revolution, Rock Band, Guitar Hero, Tony Hawk or DJ Hero, you could task your guests with putting together a costume that matches their character's in-game model. (If you are expecting guests that don't have the game, you might also put together a collection of accessories to help guests adjust their look after they arrive.)
Then again, other games have set, recognizable characters, allowing your guests to be able to easily source reference material to make their costumes or - in some cases - even buy them off the rack. Here's an idea... select a game system (or two or three) that will be playing during your party, and have your guests dress up as a character from a game for that system and bring that game with them when they come. This should provide a variety of games to be played, as well as some great costumes. Maybe limit it to "retro" games, and break out your Space Invaders ice cube makers and set out snacks that Pac-Man would eat. Or, if your group is more Nintendo-oriented, go for Super Smash Bros. Brawl and have your guests dress like any of the three thousand playable characters from that.
