Design what will be customizable. Needs to be simple in terms of required support to implement.
I would like to see:
Custom sails & flags
Custom clothing for crew members & captain
Custom camo paint for boat it's self
More you can customize you ship the more of a proud owner people will be. To be clear by custom i mean you all have a set list for people to pick from. Such as in Assassins Creed: Black Flag. You could pull a lot of ideas from there!