Though i am a castle head, it was the pirate ship which first sparked my lego hobby. The latest imperial flagship is fantastic!
What I would like to see moving forward is an ironclad ship of the line. A broadside ironclad like those fielded by the French in the 1860 is what I am thinking of. This could be made part of a pirate theme or as part of a steam punk theme.
One could even extend the concept to land battleships for a steam punk line.
EDIT here is the poll. Good suggestion!