So come on, of all the cool words that have "gun" in it, how is "shogun" not in EtG already?
I tried drawing my idea for what The Shogun would be, only to find out that fire, especially in a charging state, is hard to animate well. Sorry :( . Nonetheless, if nobody else submits something named this, then I call dibs and I'll try my best to explain what it would be.
In feudal Japan, there existed a powerful firearm called bo-hiya. It's the Japanese version of fire arrows, EXCEPT SHOT WITH PORTABLE CANNONS. Pretty cool, right? The arrows placed into the cannons were these long metal rods covered in torch-like material to keep them aflame. Like other fire arrows, the main purpose is to burn down enemy ships and buildings, but I think I could safely bet a direct hit with one would hurt quite a bit. I've included a few images of what they looked like for visual reference.
As for how the gun works in game, I would imagine that it functions similar to the Serious Cannon in clip size and charge, but without piercing/bouncing like the cannonball has. Instead, it would hit the enemy directly, doing serious damage and inflicting burn. If it misses though and hits a wall, the fire should spread out from where it hit (like a molotov but maybe not just a simple circle on the floor). The major drawback to this would of course be the long reload time and charge time, but the payoff in damage would make this fun for the player.
Anyways, I wanted to make this submission more or less to point out that "shogun" is a fantastic pun that should be in the game and the bo-hiya is a fascinating, old weapon that would make for a fun mod weapon.