Yes, we’re not expecting the whole entire game is completely mind-blowingly unique. If you can take an existing genre and add a twist, that’s plenty valid. Art is theft after all.
Kroc added on
Mary Sue is best known from the Sonic fandom. It means creating a character which has a great over-importance in an existing, established canon. Such as creating a character that is Sonic’s girlfriend. Another aspect of Mary Sues is extremes of power, such as making them as a hyper-dimensional being with the power to destroy worlds.