Actually, gunpowder already exists in regular use in the game: Bacchan Rockets. Mind you it works like the old Chinese rocket guns (read: dangerous to anyone near them, innacurate, and hard to use, but lots of fun) and they're relatively expensive, but they're out there. Mind you asside from pirates and mad sorcs no one uses them as a weapon, but they're out there.
Our group HR'd them as 2d6 fire damage to a 10' radius, DC 15 reflex for half. Range increment is 10', but they're got a special rule for misses. Firstly you always target a square, not a person, with the weapon. If you miss the DV 10 target, you make another attack roll, then you roll the scatter die (1d8) to determine which way it scattered from the target; if you hit DV 10, it lands on that square, if not then roll again and keep going until you hit something. This does take a while, but it's worth it just for the look on everyone's faces when the rocket scatters back into the guy who fired it and his other rockets cook off. (we took almost 30 minutes to figure out where the destruction rained, but heck, we had fun and, technically, won the encounter.)