Realistically, what is the biggest franchise that Microsoft can buy out that would give them first place in console sales?

[TheWPCTraveler]

You can't. Closest thing to this would be Square Enix and the network effect.

Force Square Enix to put their games on Windows Phone and Windows 8.x, with XBL-integration.
Then, you know, make the rest of their games exclusive to XBOX.