If games such as Guitar Hero and Wii Fit are allowed to use their own custom controllers, then why shouldn't Metal Gear Solid 4?
Apparently, this is what Hideo Kojima had in mind while developing the game.
MGS series producer Kenichiro Imaizumi revealed that Kojiam and his team have "tried running electric shock waves, installing a pulse reader ... we also tried to build a controller that's receptive to the strength of the [player's] grip."
"We tried a lot, even a transforming controller."
Fortunately, none of those controllers made it to the final game. Kojima has expressed his interest in Microsoft's Natal, particularly its facial recognition abilities and how it has the chance to "change peoples' lifestyles."