Introduce Shakespeare to D&D, and encourage him to popularize it.
Not only would the campaigns he ran be amazing, but goooodlord imagine the subversive effect it would have socially. Unpinning good/evil from lawful/chaotic in the public perception that early on would be a Big Deal; bringing the idea of consumer-generated content would shift attitudes to art and literature away from a purely top-down concept, and the resulting rise of Victorian fan-fiction would be so amazingly terrible it would rip its own hole in the spacetime continuum.
Yes. The idea that he wasn't real is frankly laughable. We know where he lived, we know who his wife was and what she was left in his will (their second best bed!), he met the queen for crap's sake!