1. PC
2. Don't do a subsim, the competition in SH5 is just too great and it has a massive budget, instead do a surface sim, many people are crying out for this (and you have great expertise in this area already). I would suggest making a PT-Boat or destroyer game as a simulator (ie the game that Akella had 4 years ago before "crossbreeding it" with Battlestations Midway). As a follow on from this do a Solomons game loosely based on the great "Task Force 1942" from Microprose.
3. A first person simulation game with simple "wingman" commands for the other PT boats in the squadron.
This
old poll I set up some time ago to guage members opinions on the future of the Silent Hunter series may be of some interest/use to you.