As I did in Hoi3, you can mod a lot. The biggest problem after all is the AI. If you don't have the values and triggers to really seperate the use of this new ships, then unfortunately it doesn't make sense to include them.
F.e. I modded all smaller classes below DD's in, even MTB's (motor torpedo boats).
But...the AI never used them proberly, because it can't understand the task of them. AI started to mix MTB or corvettes with carriers or battleships....a.s.o.
Maybe this all can somehow be implemented in the lua's, but this is my weakest skill. Can't do this poperly. I just can tell the AI that they're in and that it should produce......nothing more
After all, I hope the new AI is way more modderfriendly, so that you can easily teach the AI the different purpose of all these additional ship classes. Same for new airplane types.