Basic stats:
Swings faster.
Runs faster.
Random percentage of damage penalty.
No backstabs.
No crits.

Fun ideas based off the movie:
Vulnerability to fire damage.
Cannot be extinguished unless you pick up a health pack.
Permanently cloaked until you attack an enemy or get damaged by them, at which that point they can see you, but you can still take damage from outside enemy fire if they figure out that you're attacking their teammate and be able to expose you.
Your own team cannot see you.
No cloak or revolver while wearing.
You can no longer cap points or intel.

Trailer/Backstory:
Near the end of a match, Pyro chases down an enemy spy and brutally burns him to death. A few nights pass until a random halloween event happens and spy is inexplicably back again somehow, except no one knows for sure, and they can only see him clearly in their dreams. He's back to haunt the enemy team and get revenge for being burned to death, this time with a wacky overhauled playstyle, and both teams are just as confused and scared as one another.