::I wish I'd thought this through further before I did some of this categorization, but I agree that a "unique designs" cat is probably the way to go.  It would have sub-categories for battleships, aircraft carriers, etc.  Agincourt is unfortunately the only unique battleship design I've found right now, so she would be going right into another single-article category, but I don't want to link the whole "unique designs" cat and have people wondering if "HMS Agincourt (1913)" is a battleship, an aircraft carrier, or what. 
 ::On the same note, maybe a "planned classes" category would be a good idea.  For example, I put Lion class battleship into its own category, but it's the only article there and there's no potential for more.  It would probably be more appropriate to put it into a "planned battleship classes" cat.