Here are some ideas for potential rank-up requirements:

• Continue the requirement progressions from previous ranks (Blocks Placed/Broken, XP Requirements, Blocks Traveled)
• Economy stimulating requirements (eg. spend/accumulate $XX currency, sell X items to other players at a sign shop)
• Start a crew
• Build and pilot one of each type of ship from all previous ranks.
• Sink an enemy ship
• Kill X players with cannons
• Win a siege
• Sail X blocks in Movecraft ships
• Travel a very long distance on foot in one sitting.  Teleporting or using a vehicle resets the distance counter, but logging off does not.
• Brew some grog
• Perform a server infrastructure service project, and get it signed off by an admin/mod/build-team-member.
• Rent a shop at /warp shop