This idea came to me when I was playing Awakening. I wanted to use my archer, but I didn't like how even though being on the offensive with him wouldn't allow the enemy to retaliate because once it was the enemies phase, he'd be a sitting duck and wouldn't even be that much help if it's just one enemy, he'd just be there. I wished that my archer could just somehow defend himself against the enemies that go into close quarters (single square) space, and that's when I thought of this; ironically, it wasn't firing bullets, but bayonet usage that made me think of maybe using a gunman as a class in Fire Emblem. Specifically, I think a Musketeer would be the name for the base class, and Rifleman could be the name for the advanced class.
Some of you may think this may not work too well in a fantasy setting, but that's not true; in some "Sword & Sorcery" worlds, gunpowder weaponry exists along with swords, bows, magic, etc. A great example of this is Legend of Zelda.
Anyway, here's how I think a Musketeer class would be: their design would be a round hat with a feather, puffy renaissance/late medievalish sleeves, and a big/long shirt with a nation's crest on it, sort of like this: