But who is the spooky boss
The Mann Brothers.
They would each spawn near the other's base and attack the enemy team. If one of them is defeated, the other one can still be alive. They are mostly to hinder each team from pushing the cart. When the payload reaches the pit to hell, The winning brother pushed the ghost of the losing one over the edge, resulting in him falling to his doom.
In the payload race, they can also activate a temporary team-special buff for pushing the cart past the halfway point.
What do you guys think about the idea? With what I've said, I think Pipeline would work best for it, preferably the stage with pushing the carts off a cliff.