There are three things I would love to see in-game:
1) Nation specific turret designs
2) Additional light cruiser hulls
3) Addition of Naval Forts or shore-based artillery for the defense of ports
On the first subject, I would like to see nation specific turret designs. In the picture below, I am setting up a custom battle as the United States in the 1940s. Regardless of whether or not I am trying to recreate an actual ship (i.e. the Iowa, Montana, etc.) or just make up a brand new design of my own, I find that the turret designs aren't "US Style" depending on what caliber I choose. For example, if I want to put 16 inch (406mm) guns on my battleship, the gun turrets look exactly like the Japanese Battleship Yamato's main guns. If I wanted to put 14 inch guns on my battleship (as was common for US dreadnoughts and battleships such as the USS Nevada, USS Texas and the USS Arizona), the turrets look exactly like the German Battleship Bismarck's guns. If I go with 17 or 18 inch guns, I get what I assume is either a generic model or some sort of European based model. The 9 to 11 inch guns look like old-style US Heavy Cruiser turrets (ones you'd find on the Pensacola or Northampton class heavy cruisers), and the only turrets that look like US 1940s battleship turrets are the 12 and 13 inch guns, which were calibers that had long since been discontinued on US battleships. And this isn't just the case when playing as the US in a custom battle either--it doesn't matter what nation I choose to represent in a custom battle, the turret design choices are exactly the same if I choose the same year, regardless of the nation chosen. The secondary guns have the same "issue," if you want to call it that. With the same United States in 1940 scenario, I can get gun turrets that look like they belong to ships from the US, Japanese and German navies. Now, this could just be something about the gun grades (Mark 2, Mark 3, Mark 4 and Mark 5) that I don't understand because each of those guns is a different grade as well (for example: the 16 inch guns are mark 3s, the 12 and 13 inchers are mark 5s, but the 14 and 15 inch guns are mark 4s). But if it isn't, I would like to see this matter of historical accuracy resolved for, at the very least, the campaign. This issue isn't just for Battleships either, it's the same case with battlecruisers, heavy cruisers and light cruisers. And while on the subject of gun grades and that possibly affecting the look, if different gun grades make the turrets look different, why not make a whole group of nation specific turrets? For instance, the USS Arizona's turrets don't look the same as the Iowa's (maybe the Arizona's guns could be a Mark 3 and the Iowa's a Mark 5 in the US Navy family of turrets?)...just a thought.
On the second subject, more light cruiser hulls would be AMAZING. The additional heavy cruiser hulls were very nice and well done, but again, regardless of nation in a custom battle (and once you get high enough into what year) the only light cruiser hulls look like they are still from WWI or the early interwar period. Some possible light cruiser designs I could see fitting in quite well would be ones based off of the USS Cleveland/USS Brooklyn class light cruisers and the HMS Belfast (the Cleveland/Brooklyn class hull was the basis for the Baltimore and Des Moines class heavy cruisers). It would just be nice to have some more modern looking light cruisers to go with some modern heavy cruisers and battleships.
The third and final subject is one that I feel like could add variety and strategy to the game. If you are going to raid an important and large sized enemy port, it would make sense that they might have coastal artillery to help defend it, right? Essentially, these would operate like stationary case mate guns that would have varying calibers and armor values. You could choose to either destroy these gun placements, or just capture the the port and have the guns now protect YOUR new port. And if, say, you wanted bigger coastal artillery pieces or wanted to add more armor, that could add functionality to convoys as they would then have to transport the necessary materials to said port for construction/upgrade process to begin.
I know that was a lot to read and most of the things I discussed were based on arguments of historical accuracy, but I feel like in a game like this, historical accuracy would be a big factor.
I would also like to point out that I'm not a US Navy fanboy who is whining and complaining because "I can't replicate an actual ship," I'm the same way about all nations in-game, I merely used the US navy as an example...