The core game is solid. It just needs a ton more things to do.
More varied quests. Possibly some with multiple quest steps.
More open world events that make crews come together to win, or battle it out for big rewards. This could be anything from a volcano emerging, or some big sea creature attacking an outpost, or roaming AI Royal Navy ship/s, or about a million other things.
More meaningful upgrades, both for the ship and your character. It doesn't have to be complicated, but something to add flavour to your character. Weapons with unique effects, or ship upgrades that change the handling, or number of cannons, or speed etc.