The Eternals movie could add a young Thanos and this would lead to the introduction of a new Avenger in the Marvel Cinematic Universe.
Attention SPOILERS. Thanos is dead after the events of Avengers: Endgameas they finished him off twice. The first when Thor cut off his head and the second when Hombre de Hierro made him disappear with the Infinity Stones. But even so, the Mad Titan's shadow is elongated. So we could get to see him again in The Eternals (2021) with a new Avenger.
By all accounts, The Eternals existed thousands of years ago and were created when the Celestials experimented on humanity. As for now we have little information about the film, there are many theories about it. One of them claims that they will introduce Eros, Thanos's brother, and an Avenger in the comics.
This is his history in Marvel.
In the comics, Thanos and Eros share the same father, but have completely opposite views on how to deal with the universe's overpopulation problem, with the latter even joining the Avengers at one point under the name Starfox. According to the theory, the film will tell a story that spans thousands of years. One of the many flashbacks could link the group to Thanos and introduce Eros into the mix.
And if the movie really features Starfox / Eros, it is speculated that it will continue to play a fairly important role in the future of the franchise both on and off Earth. Starfox would be a powerhouse on any Marvel team. Be it the next version of Guardians of the Galaxy or another lineup of Earth's most powerful heroes. Yes Avengers 5 is in process, Marvel will have gone from Captain America (Chris Evans), Hombre de Hierro (Robert Downey Jr.), Black widow (Scarlet Johansson) and possibly Hawk Eye (Jeremy Renner). So you need new heroes that impact the fans.
Even though we're talking about The Eternals being a race of immortal super-powerful aliens, everything that happens in the MCU is linked in some way. That is why the use of the Thanos connection would be very important, since it would unite these new characters to the great villain of the first Phases.
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