As unlikely as it may appear there is at least some possibility that Thanos could return in the Marvel Cinematic Universe, it seems. This is based on a deleted scene from Avengers: Endgame that has been circulating online recently. The scene in question is an alternate version of the meeting between Hulk and the Ancient One, which presents some logic that could open the door for the Mad Titan to come back.
The scene plays out quite a bit differently than the version we saw in the theatrical cut. Instead of the Ancient One being on her toes and on guard about handing over the Time Stone, Hulk is conflicted about the mission the Avengers have undertaken to try and bring everyone back and undo Thanos' snap. The Ancient One, trying to assure Bruce that their efforts may not be in vain, says the following.
"If someone dies, they will always die. Death is irreversible, but Thanos is not. Those you've lost are not dead. They've been willed out of existence, which means they can be willed back. But it doesn't come cheap."
This makes sense within the framework of the MCU. Thanos snapped away half of all life in the universe at the end of Infinity War. The Avengers were able to bring that life back, namely Hulk, once they reassembled all of the Infinity Stones. But by that very same logic, Thanos isn't dead. He has been willed out of existence, which is a different thing. As difficult and improbable as it would be to pull off, it is theoretically possible for someone else to traverse time, gather the Infinity Stones and will Thanos back into existence.
There are a couple of important caveats to this theory. For one, it seems hard to believe that anyone would want to bring Thanos back. Half of the beings in the universe are none too happy with him for snapping them to dust, and anyone else villainous enough to hatch such a plan would surely have designs on procuring power for themselves. Why bring Thanos into it? But most importantly, this is a deleted scene. It was deleted for a reason. Directors Joe Russo and Anthony Russo reshot the scene and opted to leave this bit out of it. That is is paramount when it comes to theorizing at all based on this scene alone.
Be that as it may, the logic dictated in the scene still seems to hold water within the MCU based on what we know. Still, bringing back Thanos in any way, unless it were in the past, which could happen in a movie like Eternals, would lessen the impact of what was accomplished in Avengers: Endgame. It would make it feel like Tony Stark died for nothing. So the odds of this bearing any fruit are seemingly low. The deleted scene in question is available on the Blu-ray/DVD release, as well as on Disney+.