A look into the world of video game modding, machinima, and other fan creations. This video goes over the history, some popular examples, and interviews with several creators. Just how much of a game is yours to do what you want with? Where are the limits?
This much: https://en.wikipedia.org/wiki/Intellectual_property#Rights
Memes usually fall under fair use: https://en.wikipedia.org/wiki/Fair_use
Next question.