The key features of the game, would be ship modifications, in a different format.
Take the Predator Submarine, lets say its the standard sub. when you take it into the dock and refit it, it would have 1 engine, which can be upgraded like a car, or replaced with a better one. it would have co2 scrubbers, you can update them and the sub stays under longer, same with torpedo tubes. you can upgrade sonar, radar, the electronics. and everything has a weight. if you hit max weight, that means you have 0 cargo space. With submarines for example you can add a segment, making it longer, benefits more equipment and space, bad side, it kills stirring and lowers speed.
What rank would do is effect accuracy, fire rate, and evasion. not turning speed, turning speed is a mechanical process, evasion, fire rate and accuracy are human components, the other is mechanical. when adding sections to a ship you lose rank, for example if you make a longboat into the size of a marauder, you would lose half your rank. Some upgrades require a certain amount of rank be used
The ships would be scaled down heavily, rather than 100 ship types, customizing them would be the key. you would only need 15-20 ship types. you can for example make a sea wolf into a sea scorpion. can make a floating fortress into a triton. But you need to upgrade them ships, it isn't a quick buy and done, the ship can be upgraded for just about ever.
The labs Modify attributes, they would not create hull types, they modify existing ones, at a deep cost in resources. For example you go to the auxiliary lab, you upgrade the propellers, they make it harder to find your sub on sonar, and increases your speed, the problem your engine is to small, and need to put in a larger power plant switching from 12 piston diesel engine to a small nuclear power plant, requiring uranium, the structure to be strengthened, so it may have been 3 speed under water, with the new reactor might be 9. Than you look at weapons, it may have take 90 accuracy down to 10 accuracy on your weapons, and may have lowered your ships rank to 0. Each change would have effects on other parts of the ship. For example if sonar wasn't upgraded, your going much faster, and by the time you see the enemy they are in weapon range, firing, and your to close to escape, your ship is longer so cant turn as fast, your trapped.
Weapon Customization, so much of what is out there is tweaks of the same weapon, rather than have auxiliary slots, you can get a weapon lets say a thud cannon, you upgrade the barrel (hardened barrels) upgrade the loader (auto loader) upgrade the targeting, upgrade the round fired, upgrade the caliber of the round.
The ammo Facility, you should make the ammo, and have different types of ammo, for example uranium tipped, armor piercing, high explosive, standard, standard would be quick and simple to make, make them in batches of 100, 5-10 minutes tops, depending on the weapon (obviously missiles and torpedo's take longer than a thud cannons shell)