I think a money pit will be needed too, besides just costumizations. (Think items such as coops or cannonball crates, think gambling games such as the cardgame promoted in your books, think of temporary pets that disappear on either a character death or the ship sinking, etc)
Not everybody wants to collect all costumizations, so once they have their picks they'll be done.
Also would love to see a lot of other costumizations, like often mentioned before:
Flags - These are really, really, REALLY important.
As a seafarer (I say seafarer cause that's what we all are, we're not all pirates) the flag you pick will tell what kind of seafarer you are.
You could have the jolly roger, red flag, british navy, spanish armada, viking flags, country flags, maybe a lot more.
The red flag could indicate that a player has no mercy and is just out for the kill, the jolly roger could indicate that the player is a pirate out for the booty and not per se the killing, etc. (They probably won't always get used for what they're meant for but that doesn't matter)
Bells - The bell could be costumized to have a different sound to it, ofcourse also to look different, but that way it would have 2 purposses when changed.
Lights - Along with looking different they could also have different colors of light, ghostly blue/green for example to start with.
General Ship Costumizations - Lots of shipparts could still use costumizations. (Actually quite surprised they weren't already there since launch, along with the flags)
Flags, bells and lights being a few of them, but also; cannons, steeringwheel, anchor, crowsnest, style of captainquarters and maybe even the ladders.
There's also something missing from ships as it is, the sleeping quarters, there are no bunkbeds or how they're called.
They could also be costumized if they're ever going to excist.
Types of costumization - You could have things to do with factions like already in the game, but also add a lot more.
Take examples from Pirates of the Caribbean (Without getting sued XD), One Piece (Anime about pirates) and other well known stories and movies.
Have a ghostly style, or a true pirate styles (More black instead of light blue perhaps..?), royalty styles involving lots of jewels and gold, hunter style like someone mentioned here already (Liked a lot of his examples), and there could be a lot more. (They could also have several of each, doesn't have to be just 1, like you could have several different types of ghostship styles)
Personalize Costumizations - Too costumize your costummizations even further you could add (Not my idea but love it) dyes to the mix, so you can pick your own color for anything you buy.
Get some actual unique ships in the game by adding those choices of color, add the possibility to create your own flag or sail as well, with symbols of skulls, shipparts, bones, weapons, animals, equipment, etc.
Then people can just make their own jolly roger, or another type of flag of choice.
Hideout - Maybe even create a hideout a player could buy and costumize, make it so every island (or a lot) have a possible hideout a player can buy, making it so a player can have only 1 hideout at a given moment.
If those excist there will be a whole world of new costumizations that can be added to the game, and could be a valid money pit too!
Pet Costumizations - When pets come out a lot of people will buy them, but then everyone has the same looking pet! :O
How about we can costumize those as well, lets say the color of their fur and eyes, maybe add a secundary color to their fur, give them a bowtie, whatever a player prefers.
There should be options for those too!
That's about all I can dish out for now on the costumizations part of the game.
Maybe I'll get back on it again soon enough. ;)
Edit: Saw someone mention the pirate legend clothing...
Add more choices for that as well!
I for one hate purple, and I don't want my pirate to look like a p**p. (Though the peg leg and hook are awesome)
How about a b****s black set of clothing, or maybe even 1 or 3 more then just the purple and the black ones, give people more options! ;)
Edit #2: It would also be cool to costumize the planks, the bowsprit and to finetune the captainsquarters.