Right now game is missing 3 players ship. Sloops is too small and Galeon is too big for 3 players. I know that it was probably discussed on forums many times, but I had lot of fun designing and thinking about it.
I think next ship, that game definitely should have, is the Brig. It's 2 mast ship quite often used by the pirates. It was much easier to operate than 3 masts ships and were smaller, but still dangerous.
Other idea is that sloop as 2 players ship is little too small, so the next one could be the Ketch. It will be little bigger than the sloop, but still smaller than the brig. It should have 2 cannons per side.
This will give us 4 ships in game, depend on how many players are playing. It should not be hard thing to implement them, and it should not break balance of the game too much. Also there will be no need to do any changes in how players are distributed through sewers. Game already supports 2 player and 3 player only crews.
Imagine ketch, brig and galleon fighting together, it should look much more interesting than 2 galleons and slop. Game will feel much more complete.