Your ideas for Arson are good, but how about things just catch on fire, and colliding with the bricks that are on fire burn you? That way you can't greif builds and burn them down, your home or store being burned would be dangerous to enter when the fire spreads.
EDIT: For example, someone sets your shop on fire. It's not going to burn down and destroy bricks. Say the floor catches fire, it burns, and you cannot touch it without being injured. The fire will spread and soon you cannot even open the door without setting fire to yourself. You will lose customers because your shop will become a death trap until the fire is put out.