On the subject of pupil dilation, in the book Thinking fast and slow, Daniel kahneman explains that for any mentally taxing task pupils dilate in proportion to the difficult of the task. If the task is too difficult, they don't dilate at all.
Bit intrusive to get the pupil dilation currently, but maybe with future VR gear we can get better information and adjust video game difficulty depending on how the player is experiencing the game at the moment.