Video games aren't made to be one-offs. Especially now, the ultimate plan is always to create a brand - a brand strong enough to have players coming back year after year and lapping up sequel after sequel to the tune of millions of dollars in revenue.
The desire to create sequels isn't only rooted in cynicism and money though. Unlike movies, it's often the second game in a franchise that ends up being the best, and that's partly because the developers are able to address issues they initially didn't expect to crop up, and include all the ideas that time, money or technological limitations simply wouldn't allow the first time around.
Sadly, the games on this list weren't lucky enough to receive such a follow up. They're all awesome, but no matter how much I keep my fingers, toes and, uh, every other part of my body crossed, the gaming gods have made it so we'll never see a sequel.