I guess it's not a good idea to make a new game in underwater environments (too boring). But some of those ideas could be incorporated into this game. For example, alien bases on Earth could be located only underwater and require a specifically designed craft/ship to get there and dock to entrance. Also, in 2083, there could be some small human underwater settlements attacked by aliens, i.e. built close to deuterium mining plants for our thermonuclear reactors - most important since aliens interrupted all deliveries from Mars (in Martian underground water tanks there's much greater deuterium percentage then on Earth). Of course, all of these would require whole new, interesting research branches.