I never realized how much hasn't been added, but has been mentioned. I had a few weapon ideas of my own, but a regular musket would be awesome. But I would find additional equipment would be useful. Like a hook to navigate islands or rope into an attacking ship from the water. Or swing around ships like a pro, or pull yourself up to higher ledges on islands. I can create a list of gear that I would love to see, but is fine if I never see them come to fruition.
Portable cannon: A smaller, portable naval cannon to use when you find yourself outmanned and out-gunned. Uses cannon balls, slow reload, and reduces movement speed like carrying a treasure chest.
Harpoon rifle: A tool to navigate islands or be a surprise passenger on an unaware ship. Functions like a harpoon on ships and rowboats, but pulls you up or lowers you depending which direction you wish to go.
Boarding axe: Tired of that crew patching holes you make? But also find yourself tucking on their ship? The boarding axe will fix that! A large battle-axe sized axe used to detach those pesky boards and reopen that hole without the expense of a cannon ball. However, this beasty weapon is slow to swing and can leave you open. But...if you can score a hit, be ready for high damage! Can be blocked or dodged.
Experimental Rotary Gun [ERG!]: A 10 round, multi-barrel gun that has a slow rotation caused by pulling a crank. This heavy invention slows the pirate down like a chest or crate would, but once the crank gets going, watch the mayhem unfold! This beast functions off the ammo box and requires full restock of ammo when depleting the 10 rounds. Reload after is slow, so be sure you are someplace safe to rearm this monster of a weapon!
These are just some fun ideas...feel free to comment your opinions and views.