Well, assuming the Franky Family isn't dead, they could be the "Crew", and Luffy, Robin, Franky, etc. could be the officers.
Granted, this isn't Luffy's style, but it'd be great to see a huge ship full of people commanded by Luffy.
I seriously think that at the End of One Piece, Usopp will go solo and make a fleet of ships. He'll be like Krieg and have 5,000 men.
Speaking of that, those arches looked pretty small on the Reverse Mountain Canal. How do you think Krieg and the Marines get their bigass ships through those arches?