I also have some questions. Why does almost everything make your submarine more visible? It doesn't make any sense to me.

Why does it allow you to put multiple Snorkels on it?

What is submarine stealth module supposed to represent? Is that a stealth coating like they used historically or what?

What is the cruising range module supposed to represent? And why does it make your sub more visible?

Why does Hydrophone/Sonar make your torpedo attack higher?

I think Radar should probably increase submarine stealth somewhat. I'm pretty sure it did in real life because it allowed subs to detect enemy ships and evade them.

But I also think there was Radar equipment that allowed you to find enemy submarines that had their radar switched on. So I think maybe that the Radar should increase submarine stealth somewhat. But there should also be a Radar finding equipment for surface ships.

Can you make an aircraft carrier submarine like the I-400 Class?

Finally, can you make smaller versions of this mod that are compatible with other mods? Or something like that?

I think on surface ships there should be two Radar slots. One for air search radar and the other for surface/gunnery radar. That's how it was in real life.

Transport Subs - Impossible due to game engine.
I-400 Carrier Subs - Potentially doable but hard to depict right and useless due to game engine.
A-150 Class - You can already do this in NRM.
Faulty US Torpedoes - Not in NRM standalone at the moment, NRM in BlackICE has this included.
Snorkels/Radar - Not exclusive in NRM.

Sub Visibility - More equipment = larger hull/displacement = less stealthy
Multiple Snorkels - You can only use 1. You can place it in several slots however, but you can only have 1.
Stealth Coating - Both sonar reflection reducing design features and the use of coatings.
Range Module - Amount of fuel carried. More fuel = larger hull/displacement = less stealth
Hydrophone/Sonar Torpedo Attack - More accurate target information (including without use of periscope) = improved attack. Possibly the bonus is too high atm, but its good to encourage using up to date sonar on subs.
Radar Stealth - Possibly, but in general submarine radar set ranges were fairly short (shorter than DD ones for example, just due to height of emitter), so it should only apply against ships without radar, which we can't check for. Also later on it should actually reduce stealth once radar detectors come into service - this is also not possible due to the engine. Right now the increase in surface detection works out well enough, as higher surface detection means less change of getting found by enemy patrols. It obviously doesn't help you when in battle and submerged.
I-400 - See above.
'Smaller versions of the mod' - No. Feel free to do so yourself, anyone is free to use anything from the mod so long as you credit the original.

Search/FC Radar - Already implemented. From the 3rd level of radar and onward you have the choice of a purely search set or a search & FC set for the radar slot.

Does this mod work with the Expert AI 4.0 mod? I wonder if it's possible to make a 'Lite' version of this mod that works with other mods? Because the Ship Designer in vanilla really sucks. I can't play without this mod.

By the way I found a really useful tool called Paradoxos Mod Manager. Just want to let you know about it.

Battlecruisers and battleships seem a little too similar, I know that theoretically BCs were cheaper than BBs due to less armor and also a bit faster, but it seems odd that BCs can mount everything a BB could with the only noticable change being the armor construction module thing labeled between Battlecruiser and Battleship constructions. What ARE the differences exactly?

Would the heavy capital ship line be split into their own trees, with Dreadnoughts branching off to Battleship and Battlecruiser lines? I think having the BCs be limited to the lower end of battleship armor but given slight boosts to reliability or speed might help differentiate between the two a little more.

Also, while tinkering on heavy capitalship armament, what are the differences between the multiple turret layouts? I know that more guns per turret mean less turrets generally, like quad-gun mounts having less turrets than triple-gun mounts and the like. Do more turrets mean more reliability or manpower usage in comparison for less turrets for the same gun size and amount? Do quad or triple gun turrets pack more firepower at the cost of less armor or slower top speed? Do they affect how hard a critical hit affects the ship if say, a turret gets knocked out?

Lastly, would it be possible to make seaplane tenders from destroyer hulls? Having them mount seaplane hangers in exchange of half their deck space could make for very cheap carrier fleets. Speaking of floatplanes, would there be additional modules for them? Fighter floatplanes for increased AA ratings, ASW floatplanes for a small boost to depthcharge attacks and/or submarine spotting, and dedicated scout planes that act like the current iteration of the floatplane module that boost surface and sub detection. Perhaps researching carrier variants of aircraft could incrementally increase the stats of the floatplane modules, much like how researching more into radar and AA guns brings better tiers.