1) THAT BLOODY DEATH SCREEN!
You should be able to see what happens till you (capitan) actually dies on the boat. You can also get killed at anytime, like the other sailors.
2) FLOODED COMPARTMENTS!
Able to sustain damage like fully flooded compartments and still manage to try to save your boat and crew.
3) REALISTIC SHIP SAILING BEHAVIOR!
No more that sports car of a ship... Ships takes 10, 20 minutes to fully stop, not 5 seconds. The ships must feel like they have 5 thousand tons, cos they actually do!
4) INTERIOR SOUNDS FOR SURFACED AND DIVING CONDITIONS!
So we (modders) can create different ambient interior sounds for surfaced/underwater situations, and also for silent running situations.
5) WAVE MECHANICS/WEATHER!
In a certain way that would eliminate the pattern effects that makes the waves look like a chessboard, with the same repetitive movement, waves in blocks. She sea is where you spend 99,9% of the time while in-game, so it have to be really realistic, both over and under the surface. Instead of only 4 kinds of weather, much more (32, 64), so we can have from no-winds/mirror like sea all the way to impossible storms, strong enough to sink/damage the sub, impossible conditions to stay surfaced. Electric storms would also be very nice.
6) U-BOAT FUNCTIONALITIES!
Possibility to use everything the uboat offers... Left engine, right engine, diesel underwater (with consequences), electric motor surfaced, operate the dive planes and valves and every possible thing that the uboat offered as a feature to be more precise about your surfaced/underwater maneuverablity. Possibility to use a precise amount of ballast (maybe by pressing & holding a key)... etc.
7) MORE IMMERSIVE CAMPAIGN
Feel like you're not alone. Messages from BDU that really mean something, like operations or other stuff that you can be part of. Messages from S.O.S from any other vessel around that are trully there, so you can go on a hunt. Messages from other UBoats trying to syncronize a wolfpack on a convoy, an existing uboat porsuing an existing convoy, not just messages to make you feel you're not alone (by the way, doesn't work).
My 7 cents.
