Rocket launcher replacement is the Firework Cannon. Primary fire is launch a single firework that does regular rocket damage upon impact, while removing the splash damage. Secondary fire detonates the firework in a huge, colorful flash, causing damage based on distance away from the firework.
Also causes the enemies affected by the firework to have the screen display as though they're on fire, without causing any damage because god damn do embers sting.
Shotgun is replaced by one of those little confetti guns. Something like
this I guess. If shot by the confetti before shot by the firework, the confetti ignites, setting the enemy on fire. Causes minicrits when used with flamethrower/backburner, flaregun, flaming arrow, or firework.
Shovel is replaced by a giant steel pipe. Something like what you might make a homemade firework launcher out of.
