Here in America the past few weeks fireworks have been going off every night. If you live in heavily populated areas you probably haven't been getting a whole lot of sleep as the blasts go off into the middle of the night. Last night while trying to fall asleep between the firework blasts I was thinking about explosives and gun powder in my Dungeons and Dragons games.
I don't usually have gun powder in my games but why not? Is it too game breaking? Is it too powerful? I don't really think so but again why don't I have it in my games? This is something I am going to start having in my games only it will be extremely expensive to buy. Anything that can do as much damage as a gun or explosive should be expensive. In turn it really shouldn't be practical to use.
Gun powder should also be very rare. Whether it is a concoction only some know how to make or if it is a natural resource it should be hard to come by. If it wasn't hard to come by technology will advance really quickly with things like vehicles, or other things that come with combustion.
It's going to be hard to find any guns or know of people who know how to adequately design explosives. It would take a lot of trial and error to figure it out. Just experimenting would lead to deaths and disfigurement. Could make some interesting NPCs in your campaign though. The richest man in town could be missing his arm or hand and be a little wacky. You would have to be a little quirky to risk your life like this right?
These are all just thoughts for my own home brew campaign. If my players want to try and seek this stuff out they can. It isn't too powerful because guys can summon lightning bolts and fire out of their finger tips without consequence. It could add to some fun. Imagine Kobolds or Goblins evening the odds by hurling explosives at players. They could deal better damage and for some extra entertainment you can even have them blow themselves up to show how dangerous it can all be.
So don't be afraid to add some technology into your games. It could add some fun or even give you some interesting encounters when you are struggling to come up with a good idea for that session.