not sure if having that option to "widen your FOV" would be usefull ,realistic and practical for a game.
Personally I find it that I am eather focusing on the image on scope and in fact reducing my FOV and, well focus. Or that I am not focusing on that image at all and paying more attention to my surroundings, and for that I believe letting go of the aiming button would do just finw.
I would say that the way most games handle it (rifle in middle of screen, slight zoom and possibly bluryness in edges) is quite adequate and representative of having both eyes open, while closing your (in most cases left) eye would simply mean shifting the rifle more to the right of the screen and cutting off left edge of it slightly (something that only people with widescreen monitors should see in firstplace).