Luckily, the Eternals saga spans thousands of years, so if Thanos is indeed appearing in the movie, it’s logical to assume it will be through flashbacks. And it’s not like he’d be shoehorned in, because Thanos is closely tied with the Eternals in the comics, specifically the Titanian offshoots. He’s the son of the Eternals A’lars and Sui-San, and his brother is Eros of Titan. So Thanos is part Eternal, and the reason he looks so monstrous compared to other members of that race is because he’s also has Deviant genetics. We also visited Titan, Thanos’ homeworld, in Avengers: Infinity War, although it was long dead after being ravaged by overpopulation and scarcity of natural resources.