Granted, we havent seen much of Thanos the Mad Titan; he doesnt have much of an established history within the MCU ventures. With the exception of a brief cameo at the end of the Avengers and his appearance in Guardians of the Galaxy, he really hasnt really had much of a chance to make an impression on the MCU audience. So his promise of being the supreme cosmic villain is only a promise that thousands of comic fans know about. But thats a lot of fans and that support can give Thanos the power he needs to make that impact. With references to the cosmic cube (aka the tesseract in MCU-speak), the ground has already been broken for Thanos to demonstrate his galactic-scale evil ambitions. A wild speculation here: an obvious storyline for Avengers II is the Titans search for a source of inexhaustible power to fuel his cosmic ambitions. Clearly that search would take him to Earth. If a fan can see this, what can the screenwriters come up with? The MCU rests solidly on the expectations and loyalty of these fans. Thanos is an established villain in the comic verse, but more importantly, he is a potential treasure trove of cosmic villainy! He is an insane, warmongering lunatic who hallucinates that he is in love with a female personification of death! He is a galactic-level threat who has no inhibitions about destruction and the death it will bring. There is no end to the number of storylines this mad god can generate. He is a screenwriters best friend and is needed in the MCU very badly.