A submarine is meant to hunt and destroy targets. Since none of that took place in the cold war, and because the time period is too recent for animosity to have dissipated, I believe a cold war Silent Hunter would be a yawner. I also think that the state of submarine simulation as modeled is pretty close to the highest possible given the current state of hardware. So...
On to something completely different! We could go to WWI, but WWII had such variety of submarines and nations with submarines. I would model both German and American subs. The English, French and Japanese had subs that were very interesting as well. But all that would duplicate present efforts, so lets load up a curve-ball (apologies to non-Americans for that obscure metaphor).
I would propose an on-line game with cooperatively crewed boats, one player per crewmember (or one player per compartment in some cases like torpedo rooms). Unfilled slots would have to be able to be manned by AI NPC's (non-player characters). Wolfpacks could be modeled with multiple cooperating boats and the targets could also be manned by players too if they wanted to do some really fancy footwork. This game would incorporate both Atlantic and Pacific theaters, or could be sold as two separate games, I'll buy 'em both. It really should use the British, French and Japanese subs too.
I think this is the idea that will sell enough units to finance the next generation of submarine simulations. My bill will be in the mail. I'll accept free games as payment.