other categorical suggestions...
Environment - Mods that affect the world around you...sea, harbors, weather.
Gameplay - Mods that affect game mechanics, i.e. Longer Repair Times.
U-Boat - Skins, interiors, crew, and other mods that directly affect the U-Boat.
Ships - Ship packs, skins, etc.
Subcategories might include Allied, Axis, Kreigsmarine, Civillian, Merchant.
Just a few ideas for you to rattle around in your pointy head. :p
Hope it's helpful to you.