How about being able to buy ship upgrades? By this I mean, you'd still be in a sloop, brigantine or galleon but the starter craft would all look really shoddy. The storage barrels would all look broken, the map table would be a basic 4 legs and top, the crows nest would looks like it's been nailed together by someone who's been on the rum all day and the deck would look really badly made.
You'd have 3-5 upgrade levels to each boat class, and each time your boat would look better made, until you reached the top craft type. The top craft would have gilded, golden lanterns, have no visible signs of wear and tear, the crows nest would be have metal banding and iron rivets, the bow sprint would be a shining metal spike, the shutters would be beautifully made and the furniture inside the captains cabins would be luxurious to the extreme.
Essentially this boat upgrade type would affect all areas of the ship that the existing cosmetics system doesn't. Each upgrade would also be A LOT more expensive, say £1,000,000 for the first level and reaching £5,000,000 for the top class.
The upgrades would give you no gameplay advantage at all and they would be solely cosmetic and our current cosmetics could still be applied to each ship class.
I really like the idea of feeling like I've "bought" my ship instead of just getting given one when I play and this would make me feel more like my ship belongs to me. Obviously the ship class would have to be applied in the same way our current cosmetics are.