One of the innovations in my fantasy campaign will be gunpowder weapons. I'm doing chiefly this to show the players that sometime has passed since their old campaign. That and I had a lot of fun running them through a dungeon with mechanical monsters that was so much fun it must have been illegal somewhere. Those monsters screamed Steampunk to me and demands I address it.
Gunpowder will be not quite magical. It's made with mundane means but tends to blow itself and the maker up. Muskets and pistols will be the weapons of garrisons and armies. Bows the weapons of adventurers. Learning to load a firearm is fairly simple compared to the years of training required to really master the bow. The saying went if you wanted a good archer begin training his grandfather. Gunpowder weapons are fairly easy to produce and a musket unit is much easier to raise than a longbow unit.
In game terms a pistol does 1d8 damage and musket 1d10. Both are exploding dice. If you roll maximum damage (i.e. 8 or 10 respectively) you roll and again and possibly again and again. On the down side a pistol has an effective range of 20 feet and a musket an effective range of 50 feet. Attack is -4 for every doubling of range. A character takes 4 rounds to load a pistol and 6 rounds to load a musket (meaning you better hit, meaning you better wait till that gob is at short range ...) A fouled weapon or damp powder or faulty flints are all options to keep pc's alive. Some powder does not travel well. Only the dwarves make kerneled powder regularly. Other races consider their powder the best and it is in high demand everywhere.
Adventurers prefer the bow. Quiet, a higher rate of fire than a musket and appreciable damage at longer ranges. Still many archers were put out of jobs due to gunpowder and have turned to various enterprises to make a living (ever wonder where all those adventurers come from?) firearms have an effective range at present measured in tends of feet. The best way to use a pistol is to poke your target in the head with it. Knights are still in business. A cavalry charge is still a fearsome thing. Getting a shot in requires you to stand your ground. If you miss you get run down. Most shooters flinch.
Some people are already moving away from heavy armor (rogues!) and taking up lighter DEX dependent weapons (and packing some heat in the form of a pistol or two.)
Dwarves make excellent firearms and stable gunpowder. Orcs make standard weapons and are a real pain to adventurers now. Elves usually refuse to have any truck with guns. Gnomes relish gunpowder weapons and have all manner of repeater weapon they're developing (so far only misfires repeat reliably.) Gnolls, like the elves, will usually not use gunpowder weapons even when they are supplied. They find the stench repulsive.