I would like to see
Quadruple turrets for calibers 13-inch to 17-inch
19 & 20-inch guns (single and twin turrets only, are the same size as the 18-inch turrets)
The big one: Aircraft Carriers: Unable to mount main guns unless using a Hybrid CV hull, You select the aircraft carried and the number of each type according to a maximum number of aircraft set by the Max displacement of your carrier
AA guns (because CV)
Country specific hulls, guns, and everything