[Pisces]

Here's my take:

0. Bugs. Keep the patches comming if there are still obvious gameplay bugs present. Crash-free doesn't mean it works ok. Go the extra mile to create quality.

1. Accurate physics: Curved earth and propper rendering of it (over the horizon visuals/radar), propper newtonian mechanics(inertia and collision effects), Euler-rotational dynamics, speed of sound delays of explosions, flooding based damage/sinking, diving with user controlled-trimming/ballast (maybe realism option). Full speedrange propellor-RPM vs. screwnoise modelling. Thermal-layer simulation even if it wasn't discovered yet at the time.

2. Better technology modeling and user interface:Manual targeting as was performed, like split-prism optics AS the Germans used it (Range and AOB measurement combined). Gyrocompass synchronised Fixed-Line-Through-Space in periscope. Accurate clock readout/timezone and stopwatch use in every screen. Sextant for celestial navigation, true sun/moon/stars and planets. Keep the map and drawing tools as-is (made SH3 a winner for me), but don't forget editable text-fields in marks! War-diary/captain's log.

3. Built-in randomness in i.e. range reports from crew/sensors, based on crew fitness/experience/AI. Better yet, make the variability modable, as accurate readings are a good thing during testing! A helmsman that keeps his course.

4. Keep the Sh3 campaign type. But with Wolvepacks (AI). Make land objects just as real as scripted or random layer.

Note: nothing to add in the graphics department. Good enough, and the above is more important for immersion.