There’s really no roadmap for filmmakers after they gross over $2 billion, let alone one for those who gross that sum two years in a row. James Cameron opted for the “Avatar” ad infinitum approach, with biennial instalments scheduled to run for the next decade (“Avatar 2” premieres in December this year; “Avatar 5” comes out December 2028), but the reigning champs of Blockbuster, Joe and Anthony Russo, took a different tack.