Get rid of barbettes as a separate placeable. Instead, have each gun turret generate its own barbette. Basically, you place the turret where you want, and using the page up/ page down keys, you would be able to elevate it, while the game would automatically generate an appropriately sized barbette for each gun.
This would get rid of the visually unbecoming oversized barbettes that you see on so many ships in the game.
This feature would include secondary guns as well. For example, you might want to build superfiring 7 inch guns at the flanks of your superstructure. Secondaries could also have higher barbettes, giving us the option to have three superfiring turrets.