This isn't so much a crack theory, but a theory that was born out of speculation of CC's initial teaser images and quotes from the game.
People really believed Angeal would be revealed as the original founder of AVALANCHE, and Zack would have to be torn between following his mentor, or obeying orders and taking him down. And there was one teaser image of Zack and Angeal back to back in front of a window with the full moon overhead, and Angeal stating "Our enemies, are all those who threaten the planet."
Kinda made people think he was speaking of AVALANCHE, and trying to get Zack to join.