Also, the sim would need some exciting new feature so the crowds of buyers wouldn't think the game is just an improved SH3. Like a random mission generator, an epic campaign with an actual story or the first-person walking featured in SH5 (which was the worst feature in a subsim ever because of bugginess). A good subsim needs clear GUIs or a control panel so you don't have to walk around the boat and talk to crew members to get the sub up and running. I've also had TheGeoff's tiny nuclear subsim project in mind, the crew and sub system management is very realistic, except that its Minecraft-ish graphics are yet not tweaked for the hardcore simmers who want super-duper graphics.
