Xander is barely player character material. I don't think a game could or should model his role as a PC option.
He has no special abilities. He's a good guy, and has a heart of gold, but if Buffy was an actual roleplaying session (rather than a TV series with scripts), Xander's player would be whining about how much his character sucks. He is barely combat worthy, his non-combat skills are fairly lame, and all the other players get more spotlight time and have cooler things to do.
Buffy - Super stong, super tough, kick-ass fighter.
Willow - Very potent witch.
Angel/Spike - Vampire ('nuff said).
Giles - Tons of knowlege skills and some combat ability.
Oz - Werewolf (but still more of an NPC).
Dawn - NPC
Anya - NPC
Basically, Xander is a great character for a TV show that has script writers who can keep him alive and interesting.
But for an RPG without a script, he is a very sucky character that very few gamers would be happy to run...