What do you think it will be? Bungie said the X button in Halo will change the way we play first person shooting games forever.
What could it be? A leg coming out of the master chief/elite to kick your opponents like in Dark Messiah? Activate the chainsaw bayonet in the assault rifle (without scope, like in the original Halo!)? Does it trigger the "auto-teabagging" mode? Does it kick the 12 year old idiots?
So many possibilities.