OP - This was an excellent and well thought out post. This is the kind of constructive contributions that help drive the game forward. Haters gonna hate of course, because internet.
In general, most of your suggestions are around ways to make the game more varied, interesting, and fun. The Realism Police that run these forums aren't going to be receptive to alot of them, and will likely just point to a bunch of broken mechanics that in theory should do the things that your suggestions hope to do. But I I generally support them
My two cents on some of the concepts you brought up and the initial responses:
Crew Hiring / Crew depletion while at sea - The problem with this is that the undercrewed penalties are so steep in this game, if you don't auto regen crew you basically have to go back to port after every battle. That is no fun. My vote is that this is one area we keep the unrealistic assumption in support of keeping the game fun.
Mission Types Variety - This is a critical, necessary upgrade. I grind missions for XP like a lot of people. And as much as I love the actual combat in this game, man it gets old doing the same damn search and destroy mission hundreds of times in a row. Want to stop the hemorrhaging of the player base? Mission Variety is one of the first 3 or 4 changes that need to be implemented ASAP
Hidden and Secret Zones - I feel like the Realism Police aren't going to go for this one, but to some extent it is a good idea. Right now I have almost zero incentive to explore the map. There is this massive, gorgeous Carribean overworld to play in, but as a rational player it makes the most sense to just grind missions around the same port I do all my crafting.
There needs to be more incentive, then is currently in place, to explore the map. The economy / trading is not enough at all right now to make it worth me driving a few hours to other ports. I think there should be "Floating Treasure", "Hidden zones" or other random boons spawning across the OW that would give me some kind of reason to go out and explore. I want to sail around this map so bad, but there just is no reason now.
Captains Log Book - YEAH. This game needs consolidated stat keeping. I should be able to go to a Captain summary page and see a detailed list of all my kills / assists, shooting %, and all kinds of other interesting metrics. This is so easy from a programming perspective, all these variables already exist, they are just not being stored / reported out explicitly. And they really should be.
Talent Tree - Why not?? People above me seem to be freaking out about this taking away from the "Skill" component. That does not have to be the case at all. You just make the benefits of specialization / talent trees a little less powerful. They aren't more heavily weighted then Skill in combat, but does give tiny little advantages in the areas that you specify. The trick is just don't make the Tier benefits overly powerful, and boom, you have a fun system