His origins are not relevant since through the decades he has spread his influence through many of the Spheres and now is a major force in many more than just Greyhawk. It has been written in as Canon as well.

A Greyhawk setting based movie wouldn't be so bad as far as I'm concerned, but it might not be as well received by the general public as it hasn't gotten the same level of attention as the Forgotten Realms has over the years.