For those unfamiliar with the Eternals, they are based on comics created by Jack Kirby on a race of beings who were humans that were experimented on by cosmic being known as the Celestials. This resulted in two different races, the Eternals who are generally viewed as the good guys, and the Deviants who are typically seen as the bad guys. Fun fact: Thanos was born to Eternals but carriers the Deviant gene, which explains why he looks the way he does.
It is unclear as to what the movie could be about and how faithfully it might follow the comics, but there was a line uttered by Thanos in Avengers: Infinity War that could be something worth exploring should the movie come to fruition. Marvel’s future plans for the MCU are a bit murky at this point in time.
We know that there are several sequels planned for existing movies, such as Black Panther, Doctor Strange, and Spider-Man, but beyond that it is unclear what Marvel’s next step could be. However the company has stated that they will reveal plans for phase 4 come 2019 where presumably they could be waiting to see if Disney’s acquisition of Fox (and subsequently the X-Men, Fantastic Four, and Deadpool) is successful.