Mild spoilers ahead for Avengers: Endgame. With a three-hour run-time, there's plenty to mine from Avengers: Endgame. But one line of dialogue you probably missed might just tease the arrival of a long-standing Marvel Comics character: Namor the Sub-Mariner. That three-hour run-time is fairly lean, meaning that few if any lines of dialogue included in the final cut are throwaways; there's a reason they made it through the last editing rounds. And this wouldn't be the first time the MCU has dared to drop the most subtle of teasers for their …