One thing I don't want: one second of dev time spent on making a pseudo-FPS full 3d navigable interior if it means less time working on the core simulation and fixing bugs.
One more thing I don't want: attempting to be hardcore by not letting the player save and exit whenever desired. I don't care if I'm on the surface with a DD bearing down on me, I have other obligations and other uses for my computer. It's damn frustrating that I have to ALT-F4 and risk SH3's famous career-corrupting bugs because the game doesn't give me the option to exit if I'm engaged. On the same note, if I want to reload and play an old save that should be my decision, not the devs. Auto-deleting all later saves is just arrogant stupidity.
Also, no 4-engine bombers doing lob-toss attacks like modern high-performance attack planes with computerized targeting systems.
Now what I would like: better crew management, better fatigue modelling, morale, ability to set watches or have the XO manage the crew. Anything that makes the crew more individual and not just a collection of qualifications and XP would be nice.
Better damage modelling for all game objects. Sub damage affecting noise and drag. Better sonar/hydrophone performance modelling with thermal layers. None of this being detected because the DD is "elite" and you're a certain distance away under conditions you'd never be detected in IRL.
More interesting missions. A real dynamic campaign. Random != dynamic. Response to the player's actions = dynamic. I don't need to singlehandedly win the war, but if I sink a convoy carrying vital fuel and aircraft parts it would make sense to have enemy air activity hampered for a while. If I sink a bunch of escorts it would make sense to have less escorts available in the area until they can be replaced. Likewise, if I fail to sink the enemy they should be able to interdict my supplies and make fewer upgrades or reloads available. The standards for what constitutes a dynamic campaign seems to have dropped a long way since the days of Falcon and EF2000.