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.
That's interesting. I could imagine a video game to employ this somehow, as basically the difficulty in designing video games is trying to make the game not too hard or too easy, so the player gets a challenge but doesn't experience it to be too hard.
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.
There is a theoretical risk of player mind spotting the causal link and abusing it. I cannot say definitely is it possible or not, but if pupils dilated not only due to complexity of the task but for other reasons too, then it would seem completely plausible for me.
