What one game would you give a sequel to?

This is the assumption that the game you're talking about is a game that never got a sequel in the first place. I'm talking Half-Life 2 style, the series just ended on some kind of cliffhanger or something similar. Which game do you feel really deserves a sequel? When you're considering your choices, let's throw something else into the mix. When you're (obviously) getting your game back on its feet and ready to to live, just imagine you've got all the original developers/voice actors/assets and so on. You've also been generously given the budget of a Triple AAA game, such as GTA 5, to do so as you wish. 

So, with that in mind, what game is it and what would you do with that game to make sure the sequel is a massive success?