With the release of Cursed sails and a new ship this post is very relevant again. I'm glad we got the Brigantine coming and I believe only 2 more ships are needed to round out the game.
I still say a 1 person catboat would be great. One cannon on a swivel mount. To make it not OP have it behind the mast so that firing straight ahead will damage your own ship. Approx 270 degree radius otherwise. one fixed cannonball crate that holds 50 cannons. Rudder with no wheel. one gaff rigged sail that angle and raise/lower from one control. Gaff rig should make tacking super fast even when compared to the sloop. Doesn't have to be faster than the sloop but should be a great deal faster to change direction. Hard to hit but two hits pretty much guarantees a sinking and no place to hide loot.
The Man o' War I still think should be unlocked to captain as a Pirate Legend. As in only PLs can attain it and you can't queue up for one randomly unless invited. (could recruit from the server) . It would give PL's something fun to work toward and as the ship is a defensive ship mainly for long journeys it wouldn't be a ship to hunt other players with. I really think this is the best way to have a ship larger than the galleon as it should be a rare site and not the norm.
The Chinese junk should be a model you can apply to say the Brigantine like any other customization. Hull type instead of color. Would give a whole other feel to the game having something new to look at. And maybe the option to skin the Galleon as a Caravel so it's also lateen rigged. everything else would be the same mechanics wise, just "new and interesting ways" to express yourself through your ship customization.