In its current state it would only be an accessory to assist the game to be more dynamic and "perfect", but not for a controller. All it does it pick up on attention span and emotions. You would be an emotional wreck in a mental institute after one game if you try to control a character with it. There are other devices for that.
On the flip side: A gamer is using it. The game picks up that its getting boring, so it throws some enemies at him. Tests for scare tolerance now and then picking up the pace and fear or pulling back, depending on the gamer's emotional state. Adjusts to give the right amount of joy, fear, sadness, excitement, betrayal, ect... to basically make a game that is perfect for any and every player, every time. Replay value? Near infinite.
Hehe, now imagine it for monetization? Adjusts texts and imagery based on how you felt from the previous ads. Do it until you buy. Adjusts game to give you that mental state of purchasing, so you will buy some more...and more...and more.
I better shut up. Some company actually might do the ads bit....