To expand on an Idea I saw here I think, where Xander goes as a nobleman to go along with Buffy's and turns into the black dragon guy from WOW (Daval Prestor).
The gang gets a box of old time costumes that the Principle is forcing them to wear (could be due to the mayor's influence or or just plain bad luck) and each of the Scooby gang gets turned into some one crazy, really dark but not necessarily evil, or evil only with a theme. The costume's are originally for normal people, but instead they get possessed by:
Buffy's mom, Dawn, Jana, Cordelia, and Harmony could be included as well.
-Xander as a Dragon Age Origins version of Daval prestor. Xander doesn't absorb Daval's mind due to it being so alien to his own, but it's always there trying to drive him crazy and evil.
-Buffy as a upper end warrior type, only noble (to fit with her dress). Her slayer powers could be stronger here with more ambient magic around the world. Whoever it is must temper her attitiude, show her that killing people is the same as killing demons (some are good, some are better off dead)and teach her how to be a leader or have the ability to stop trying to be one if not. Possibly a slightly evil version of the female warrior Main Hero.
-Buffy's mom: Flemmeth from Dragon Age Origins
-Dawn Morrigan from Dragon Age Origins
-Giles as the Crazy Hermit (who steals the acorn) From Dragon Age Origins.
-Willow as the Hero's companion/(vassal?) assasin Zevran. This fits more as I see Willow as being loyal to Buffy and betraying her old associates (in this case Xander) whereas Zevran betrayed his former employer(owners?) and associates.
And just because:
-Andrew as a Mabari war dog (who stays that way afterward because he freaking deserves it, as a result he goes a little nuts due to his mind breaking) because he looses a bet with the other 2 idiots. Be creative with his costume, lol. One of the caster's could trun him into one when they see him acting like one. Since they never reverse it, it sticks.
Maybe sometime after Halloween (gotta give them time to screw up lots of plans) someone opens a portal and dumps them in a universe that there costumes could have come from. AU DA:O-verse if you like.
In the end none of them should go evil though just not shiny good anymore. Some will struggle more than others to keep from turning evil. many of their behaviors, morals, and/or Moray's will change (at least slightly) though.
I'm only about 20% of the way through the game so there maybe better choices I haven't seen yet.
Cracky Idea: When they land In DA:O someplace Xander ends up somehow with control over several darkspawn who he tries to control but won't do what he wants them to simply because they are interpreting what he says through their own twisted minds. If he tells them to be well behaved, they do so to the best of their ability, unfortunately being well behaved for them means lots of mischief, misery, and violence for others. Things of that nature.