I think that you need to look a bit further than what we already have or just repeating it in a different era. As far as submarines/u-boats are concerned, the best and most interesting theatre is German U-Boats in the Atlantic. So, what do you need to do to have the "next generation" of simulators.
I believe there are two aspects.
Reactionary campaign
Yes, the dynamic campaign is great, but let's take it a step further and make it reactionary. That is, through lectures, leading by example, or just contributing to tonnage, you can alter the turn of events in the war. Not by much (I mean, one U-Boat can't compete with the entire Ostfront), but still, success would cause
your flotilla to excel above the others. Also, if you were operating in a particular area, then that area would attract more attention with time. And, if there were "special mission", you would be involved in dropping off agents, setting other weather stations, laying mines, and so on, which in turn would earn you additional status and the allies would react to your successes.
Grand-scale Multiplayer
From what others have written, player vs player is essential. However, let's expand the options a bit.
- U-Boats (a given, but let's make it possible to put multiple players in a single boat)
- Destroyers and other escorts (a given, and like the U-Boats, multi-positions)
- Britisch coastal command (put in flyable Sunderlands, Wellingtons, etc. make it so you could take any position in these aircraft too - pilot, bomber, gunner)
I would stop there, to make the project feasible and to stay with "Silent Hunter". But, one day, there will be "Battle of the Atlantik" with surface fleets and so on too. That would not be "SH" though.