Recently, the Celestials have been playing important roles in the Marvel Cinematic Universe especially after the events of Eternals set them up to become more vital figures in the future of the MCU. They remain some of the most important forces in the history of the Marvel cosmology, and their creator could end up returning down the line.
Serving as the overarching threat of Al Ewing's run on The Ultimates before making its formal debut in Ultimates 2 #5 (by Al Ewing, and Travel Foreman), the First Firmament is one of the oldest beings in the entire history of the Marvel Universe -- and could become a notable threat to the MCU as it introduces more cosmic elements.
As the Ultimates discovered during their time trying to protect the universe in the aftermath of Secret Wars, creation has been destroyed and reborn multiple times. The first representation of the original Marvel universe was known as the First Firmament, a precursor to Eternity. Lonely in the vastness of existence, the First Firmament created two beings -- the Aspirants and the Celestials. While the Aspirants were loyal to the First Firmament, the Celestials rebelled, seeking to create their own future. Conflict broke out between the children of the cosmos, leading to the Celestial War. The conflict ended with the Celestials triumphant, and the First Firmament fled to a place outside space and time with what remained of the Aspirants.
Meanwhile, the remnants of the First Firmament's power transformed into a new being known as the Second Cosmos, who created the multiverse. Over untold billions of years, cycles of entropy and rebirth resulted in the multiverse dying and being reborn again and again. With each one, a new incarnation of Eternity was born. The seventh multiverse was the one that housed the original Marvel Universe and its multiverse. However, the seventh multiverse was effectively saved and restored to how it had been before -- resulting in the continuation of the seventh cosmos and Eternity. This allowed for groups like the Ultimates to radically alter manifestations of the galaxy, such as remaking Galactus into a Life-Bringer instead of a World Consumer.
But this also meant that Eternity -- still the living representation of the multiverse -- was weakened enough to be constrained. The First Firmament saw this and returned to reality, capturing Eternity and feeding off of his essence. This subplot became one of the most important underlining plots of Ultimates and Ultimates 2, as the First Firmament regretted its creations and sought to become the only being in existence. The returning Aspirants were able to sway Lord Chaos and Master Order to their side, resulting in the death of the Living Tribunal. Making matters worse, the Maker attempted to strengthen Eternity by collapsing the multiverse into a single reality -- which in turn made the First Firmament's plans even easier.
Although the Ultimates were able to work with Galactus' Eternity Watch (a team of cosmic-level forces) and the Fifth Host of Celestials to defeat the Aspirants and the corrupted Chaos and Order, the First Firmament was on an entirely different power level. Luckily, the cosmic victories of the Ultimates and the Eternity Watch empowered Eternity enough to summon the other lost representations of existence from the other multiverses. Ultimately, they -- with the aid of the One Above All -- were able to lead the First Firmament into the Land of Couldn't Be-Shouldn't Be, a realm outside of all creation, where they could potentially help the First Firmament heal and grow.
With the Celestials increasingly becoming vital to the history of the Marvel Universe outside of the comics -- notably appearing in The Eternals and setting up their judgments as some of the most important overarching storylines going forward in the MCU -- it's possible that the story of the First Firmament will eventually be explored. The return of the First Firmament might be one of the most grandiose stories the MCU could ever tackle, and the return of the Aspirants could be one of the only threats in the Marvel cosmology that could be powerful enough to stand up to the Celestials. If the MCU is really embracing the Celestials as a major fixture of their universe, then eventually adapting the First Firmament could be the ultimate direction for the films.