Snowball fights....
Bring some stupid fun to PvP (or make it a instance)
Ideas:
Can only throw snowballs. Maybe a basic attack and a "heavy" attack
10 Hits and you turn into a snowman others remaining on your team can hide behind. (make sure you take care of those pesky roes first XP)
LB is a 5 hit Snowball AOE, 5 hit rapid fire (or use this as a cooldown on top of a basic throw)
Theoretically could use the Shatter map just make adjustments, snowbanks, etc...
Me and my friends got this idea from the snow drifts from last years holiday where it was extremely satisfying throwing snowballs at each other for long periods of time.