I really wanna see Usopp have great marksmen skills in everything that involves aiming. Guns, cannons, throwing knives, etc. Of course Oda would play it off and most likely have it as a gag but I still think it would be really cool to have him be really good at all of that stuff.
I really want Usopp to start using guns. He may have the farthest range (or close to it) of all the snipers in One Piece, but he isn't as fast as others who use pistols and rifles. So, in a close range one on one battle against an accomplished sniper, he'd get shot before he can draw his slingshot. So he really needs a secondary firearm in the form of a pistol or rifle IMO. Also want him to sink battleships with cannons instead of the crew relying on their brute strength.