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.