It is an interesting thought, that someone who knows everything would be incapable of ever creating something truly new, with all of their ideas naturally matching up with something they already know, but couldn't it be said that ordinary humans are the same? We can't create things without inspiration, and everything made is built on the foundations of what came before. "Originality" is more an ideal than something truly concrete, be you man or god.
So just write that fanfiction, nobody will care unless it's some really pretentious thing about Harry Potter "explaining" magic with a extremely poor understanding of quantum mechanics, because then people will unironically call you a cult leader and write several thousand word epics about how much you suck.