Celestials could be the main villain
Unfortunately for the Avengers, Guardians, and any upcoming MCU teams, Thanos isn’t the most dangerous foe in the universe. We’ve already seen one heinous Celestial in the form of Ego, the Living Planet. Ego’s non-existent parenting skills aren’t the most threatening skills the overpowering Celestials can hone.
Given Celestials have created multiple branches of life itself and formed parts of the universe, they are basically villainous deities. As CBR notes, leaked process artwork for Avengers: Endgame depict Celestials, which could hint that they’re a more immediate threat in the upcoming film or directly after the heroes defeat Thanos.
However, the concept artwork might not be the only clues that Celestials are a more noteworthy force in the MCU. A Redditor suggests that a specific robot in Thor: Ragnarok looks eerily similar to a Celestial in the Avengers 4 art. Although dialogue in Guardians of the Galaxy mentions that the Celestials (aside from Ego) left this universe, the robot’s likeness to an existing Celestial could show that the creators have visited this universe intermittently. Or it could confirm a long-standing theory that the Grandmaster is a Celestial, which would explain his homage to his fellow brethren… just in robot-form.
Avengers: Endgame will predominantly focus on Thanos and close his arc in the MCU for a moment in time (after all, his comic counterpart takes issue with staying dead). However, Celestial motifs could be hidden in the background of the film or put in the forefront of a mid-credit scene.