Slash said in an interview that the Devil is the other "real" boss character.
This leads me to believe that's either sarcastic, or Dave Grohl as the Devil from Tenacious D and the Pick of Destiny will be the boss.
Considering that, one could have a versus match between the Devil and the Grim Ripper.
It's like when he went to Georgia, but with the GRIM REAPER instead of Johnny.