How about an ignition system that works better in the rain than flint & frizzen. And then somebody ought to invent a self-contained envelope for all the powder and the ball with the new igniting doohicky attached right to it. Maybe even a way to open up the back of the gun so it could be put in the back so we don't have to push it in from the front and we could do away with the patch. Hey! Then we could make a box underneath the barrel to hold five or ten of these thinghys. And we could hook a telescope on the top so we could see farther and have to "hunt" less.
Being a smarta$$, of course, but for a lot of us everything we need was invented before 1820. The rest just adds complication and detracts from the enjoyment. Of course, if you make about anything folks will buy it. There's catalogs full of such paraphenalia. The sky's the limit.
You could get busy on a very accurate lube, made from a minimum of cheap, natural ingredients commonly available in 1750 (that still are - no whale oil for instance) that doesn't require wiping for 10 shots or so. ::