The history of the Mad Titan is deeply interconnected with the main cast of the movie in the comics.
Then the Mad Titan Thanos appeared in Avengers: Infinity War, and suddenly the franchise had an amazing complex villain that rivaled Darth Vader in terms of villainy and charisma.
“I believe I heard some rumors that he’s [Thanos] coming back in different things.
It would make a lot of sense for Thanos to show up in Eternals, however briefly.