I'm game, though what is the power origin then? Is it an inborn human gene that skated under the radar till now? You mentioned it wasn't supernatural, mutant, or aliens.
Literally no one has a clue. :) And my apologies, because I did not make myself very clear: by "No mutants, no aliens, no ancient mythological beings," I meant that at the beginning of the game, there's no record of genetic deviation that would cause super-powers, there has been no documented contact of Earth by aliens, and no mythological beings are known to be walking around.
For reasons of plot:
I would not allow a "mutant" character, in the sense that the character has some sort of genetic alteration that has been pinpointed as the source of her powers.
A character could be an alien, but he'd either have to keep that fact a secret, or deal with the considerable repercussions of being the first alien known to Earth.
I would not allow a character to be some sort of mythological creature with a clear memory of ancient events and relationships with other mythological beings. However, if someone transformed into, say, a quetzalcoatl, and concluded without corraborative evidence that he was actually the incarnation of the legend, I could work with that. :)