*Warning, this post contains spoilers for Avengers: Endgame.
In Avengers: Endgame, we see 2014 Thanos turning to dust with his army. The story of the infinity stones has most probably ended. The main timeline Thanos was also beheaded by Thor. So unless the movies are set in the past timeline, there is no possibility for Thanos to return in the future Marvel Movies.
Luckily for us (or not..depends on whether you like the purple guy), The Eternals is an upcoming Marvel movie in Phase 4. Even though Thanos, the mad Titan, is said to be the last Eternal, he has the Deviant gene. The Eternals are the result of an experiment conducted by the Celestials on Proto-humanity about 5000 years ago.
In this experiment, the Celestials created two races of superhumans: The Eternals and The Deviants. They’re basically immune to aging, have extraordinary powers, resistant to diseases and almost impossible to kill.
There is no update from the MCU whether Thanos’ presence in the upcoming Marvel movies. So, we might or might not see Thanos in the upcoming Marvel movies, we are sure about to learn Thanos’ history and origin through the Eternals movie of Phase 4.
But in an interview, Josh Brolin explained that although his contract was a per movie contract and has ended with the release of Avengers Endgame, he is happy to reprise his role as Thanos. He said that he is open for negotiation with Disney if they plan of expanding his contract.
The upcoming Eternals Movie in which has been confirmed for Phase 4 will majorly feature the Eternals and their evil counterparts the Deviants. Disney has also confirmed that Angelina Jolie where she is going to star as Sersi. So according to our theory if Disney and Marvel want to bring back Josh Brolin as Thanos they could show a proper back story of Thanos and how he to come to be known as the Mad Titan.
So it would be very interesting what step Disney and Marvel take for the Future of Thanos.