Firework Gun Gamepass.
Cost: [up to the devs]. It would probably be more expensive than the other gun gamepasses because it is more powerful.

Each of the gamepasses solve a problem.

The Mauser C96 gives the player a decent handgun for long-medium range.
Extra Storage lets the player carry more loot.
Sawed-Off Shotgun is good at close-range.
The Thompson is good for taking out large swarms of weak enemies like Skeletons, Vampires and Zombies.

Adding a gamepass for the firework gun would give the players a something to deal with heavier and more challenging enemies, like Werewolves and Horsemen.
(I added some graphics for the gamepass in attached files)