silent hunter series seems to have a specific era in mind, that being ww2. Anything past that would be great but it wouldnt fit the SH franchise profile. Also given that at least in my oppinion that ww2 sub technology is far more interesting tacticaly speaking I would think that the next SH would again be an upgrade to the ww2 theme. Granted a cold war scenario would be cool in its own right but to me it just wouldnt be SH.
Honestly If I were Ubi I would throw III and IV together in an all inclusive package
Fix alot of AI problems (wolfpacks and Destroyer tactics) and add Italians or British or Japanese. Update the graphics engine used in III and IV to the max. Hire the GWX team to due their magic and "Viola" You now have SHV!