We ran a rumor in mid April that The Eternal was the next Marvel movie that they were going to actively work on. Kevin Feige went on to confirm the movie eleven days later. We haven't heard much since then but it seems that things are moving forward. According to The Hollywood Reporter the studio has hired Matthew and Ryan Firpo to write the script.
In comics lore, the saga of the Eternals began millions of years ago when the cosmic beings known as the Celestials genetically experimented on humans, creating the super-powered and near-immortal beings and a more monstrous off-shoot known as the Deviants. The two races moved behind the scenes throughout human history while also engaging in Game of Thrones-style cosmic powerplays.
We know almost nothing about the next phase of Marvel movies after Avengers 4. We don't even know what kind of state the universe is going to be after this next Avengers movie. It's interesting though that this is the direction that Marvel has decided to go moving forward. This isn't any weirder than Guardians of the Galaxy and they made that work. There's no reason to doubt they couldn't make this work either.