Those who pay close attention to Marvel Comics may know that in the main version of the Marvel universe, Thanos was born to two Eternals named A’lars and Sui-San. While neither of these characters are known to be included in the upcoming Eternals film, another Eternal named Thena (played by Angelina Jolie) is Thanos’ cousin in the comics.
According to a section of the Avengers S.T.A.T.I.O.N. showcase, the MCU’s Thanos has ties to the Eternals, a species of MCU characters that just so happens to have a self-titled movie set for release in 2021. Specifically, per in-universe intel provided by Thor, Thanos is “the last of an ancient and highly sophisticated species known throughout the universe as ‘The Eternals’.”
“Thanos’ origin is said to be on Saturn’s largest moon, Titan, making him a Titanian in nature. It is rumored however that Thanos is the last of an ancient and highly sophisticated species known throughout the universe as ‘The Eternals’.”
It’s been speculated that Thanos’ connection of the new-to-the-MCU Eternals could be a natural way for the latter to be introduced to the movie universe. However, this note at Avengers S.T.A.T.I.O.N. is the closest thing to an official source that has indicated this being a real possibility.