Gozer (also known as Gozer the Gozerian, Gozer the Destructor, Gozer the Traveler, Volguus Zildrohar and Lord of the Sebouillia) is the main antagonist of Ghostbusters.
Vote which fictional character would naturally fit in-universe with a big-named franchise! No time-travel, no interdimensional crap, no "Pooh's Adventures", they would just exist in-universe.
