I would love to see plenty of crew, sailors, and marines running about my deck during battle. One of the main driving factors behind a game is immersion and I feel like an interactive crew that responds to simple gun, sail, and survive commands would be incredibly immersive and would make you feel like your ship is alive and functional.
That, and the interactiveness of gun ports, sails, and steering (like the rudder) would be great too. See the way Empire: Total War manages crews in their sea battles.
I would vote for this a thouasand times over. Please add more visual crew!