Historical Campaign where player Designate their ship's nation and so on...
To designate a ship's Nation is optional and allows the ship to be used for a Historical Campaign, But this restricts the weapon and blocks allowed.
Preferably only Choosing Axis or Allies to save your effort:
Axis campaigns mostly includes convoy raiding and so on
Allies campaign mainly includes defending convoy
However you could divide it into 4:
Pacific missions includes large scale confrontations such as the sinking of Yamato, battle of Midway and so forth
Atlantic missions are small scale confrontations and mostly convoy raiding.
Missions may have certain requirements such as the player's ship must have more than 1 aircraft
if the Requirements are not met, the mission blacks out opening the next mission and the most likely/historical outcome will be applied
You could make this a DLC or a unlockable content but please don't...
Now onto minor things:
AA Accuracy is... indescribably bad...
Turreted Torpedo launchers that are small...
Mitsubishi A5M... I DESPRATELY NEED IT
Big Bombers like the B-25 Mitchell? aboad the Hornet or Wasp class carrier.