[Chromatix]

If you do manage to make it "open world" rather than "discrete mission" based, something fundamental will be the world model. Silent Hunter - all of them - use "plate carrée", which badly distorts both distance and direction anywhere but the Equator.

With modern computing power, it should easily be feasible to use a true spheroidal model, but that'll need to be designed in at an early stage. An obvious approach would be to use an Earth-centred physics model, in double-precision, and for rendering, merely translate all objects from that space to a single-precision graphics space centred on the camera. This of course means that ships in the vicinity of southern Australia really will be upside-down in physics space, but since the camera will be as well, it'll look right in the end.

A side benefit of a true spheroidal model is that draw distance decisions are greatly simplified: you draw all the ships and other objects that are above the local horizon from the camera's perspective. So it's no longer necessary to arbitrarily cut things off at a specific distance.

Extending the game world beyond the Baltic Sea also means that all those finicky details about saltwater density, tides and suchlike suddenly become a lot more relevant.

Weather patterns also vary a lot worldwide - perhaps a complete forecast-quality weather model is too much to ask from a small team, but you should at least model the trade winds, average fog and rainfall incidence, and seasonal storm likelihood, for which first-order data is available on ocean pilotage charts. Also, the smaller types of waves (high-frequency components of the Fourier transform) react a whole lot quicker to changes in wind speed and direction than the big swells (low-frequency) do. This would be a big improvement over the abruptly-changing, context-free, completely random weather in Silent Hunter.

If you plan to model torpedo failures realistically, the magnetic detonators were apparently affected by latitude due to the vertical angle of the geomagnetic field. Ambient sea noises, especially biologics and ice cracking, are also region-dependent, while the incidence of suspended silt and/or vegetable matter in some regions has a strong attenuation effect on sonar transmission, making both submarines and escorts almost completely deaf.

The above factors are the sort of thing which add to game depth rather than complexity as seen by the player; this is generally a good thing. How many of them can be done within the new game's format and budget remains to be seen.