Re: My wishlist item would be ...

Realistic proximity and radio voice chat

Radio waves don't penetrate water very well, and sound waves have plenty of trouble with heavy steel bulkheads. Barotrauma with these features and the right mix of people aboard the sub could make for an incredible experience.

Prox voice would be limited to the room you're in, unless a door is left open. It would be muffled considerably through walls and doors, depending on material thickness. Prox chat underwater should be virtually unintelligible.

Radio voice chat could take a number of forms. A microphone and speaker could be wired to a radio transceiver, allowing players near a microphone to communicate over an intercom system. This is where radio frequencies would have to come in and complicate things, for better or worse. headset radios would have quite a short range, but probably be sufficient for all but the largest vessels.

For inter-submarine communication, I see two options. You could use the existing radiocom that we already have in the game, or introduce something like ELF (extremely low frequency) communication. This frequency range caps out at an incredibly slow 300 Hz, but allows communication across hundreds of meters of seawater. This also raises another interesting situation-- subs cannot communicate at all beyond that range