My view is MOST of what's needed in an Atlantic WWII subsim was present in the modded SHIII (NYGM or GWX).
Realism, realism, realism....with SOME compromises for gameplay/resource constraints.
Let's face it:
- assigned orders to patrol zones
- dynamic contact reports
- potential for wolfpack actions
- correct sensor performances based on technical specs, experience and weather.
- crew rosters/assignments
- weather
- observations leading to plotting leading to firing solution.
- weapons (performance, especially for guns)
- damage modelling
Get those 'realistic', by which I mean a reasonable representation based on the abundant real-life records, and you're a LONG way to making a great sim.
THEN make it look nice.
IF SH5 is SHIII in SH4+ engine without addressing all those points then

.
Some simple tests I want to see are:
- surface attacks at night
- detection/evasion, surfaced and submerged
- selectable 'contact frequency' (i.e. yippee shoot, moderate, realistic)
Get those right and I'm pretty sold. Another way of looking at it is asking "what did NYGM/GWX attempt to 'fix'?" and then SH5 should have those 'fixed' at time of release.
Cheers
(p.s. I posted this in another thread and was feeling lazy so i just cc + pasted)