A host of character descriptions from Chloe Zhao's The Eternals may reveal the film's characters, including Thanos' brother.
That Hashtag Show dropped reported breakdowns for several characters, including Piper, Elysius, Gilgamesh, Ikaris, Sersi, Makkari, Thena, Zuras, and Eros of Titan. Also known as Starfox, Eros is the brother of Thanos. The breakdowns also call for a character named Karen and an unnamed Greek God that the article speculates could be Hercules. The article also indicates that Druig will likely be the movie's villain.
The casting descriptions show that the film is open to casting actors of a variety of ages and ethnic backgrounds. Looking a little closer at the descriptions, it sounds like Piper is a gender-flipped version of Sprite. Strangely, there are no descriptions for any of the characters known as the Deviants, who are depicted as the villainous opposition of the Eternals. As The Eternals is still quite early in production, there's a good chance these breakdowns won't perfectly reflect the characters that appear in the film.
Under the name of Starfox, Eros of Titan was first introduced by Jim Starlin in 1973's Iron Man #55, which also contains the first appearance of Thanos. Much like the rest of society, Starfox was shown to be opposed to his brother's plans. Eros could be a major addition to the film, though it's possible that his backstory and relationship to Thanos might undergo some changes.
Names like Ikaris, Sersi and Makkari are staples of the Eternals roster, while the inclusion of Eros could hint at Josh Brolin returning as the Mad Titan after Avengers 4 in 2019. Brolin has already said that he'd like to reprise his role as Thanos for The Eternals, but that will probably depend a lot on the events of Avengers 4 and the time period in which The Eternals takes place.
The Eternals were famously created by Jack Kirby after he created the New Gods at DC. Writers Matthew and Ryan Firpo are apparently taking their inspiration from Kirby's original run of The Eternals, not Neil Gaiman's miniseries from 2006.
The MCU saw a great success when James Gunn brought the Guardians of the Galaxy to life in Phase Two, and it looks like the Eternals could be the next big franchise. Considering Guardians of the Galaxy Vol. 3 is on hold indefinitely, sci-fi fans can rest assured that the MCU is only starting to explore the stars.