There are loads of enticing possibilities for the next powerful villain the Avengers will challenge in future MCU movies. The Eternals will introduce immortal beings tasked with protecting humanity from their villainous counterparts, the Deviants. There's evidence to suggest the Deviants could be the next major threat to Earth's Mightiest Heroes, but some even more powerful entities would be natural antagonists for Avengers 5.
First introduced in the 1976 comic Eternals #2, The Celestials are essentially the mythological Titans of the Marvel universe. They’re ancient entities with immense and seemingly omnipotent powers and are responsible for creating the Eternals and the Deviants, both of whom were early experiments in the evolutionary process on Earth.
Occasionally referred to as space gods, the Celestials are known for their obsession with the Infinity Stones. While they're not exactly prominent characters in the MCU, their presence has been established on the periphery of major events affecting the Avengers and their allies.
Ego the Living Planet, aka Star-Lord’s dad, is a Celestial. That alone is pretty solid proof that these powerful beings have been to Earth before. The original Guardians of the Galaxy movie also briefly mentions a Celestial known as Eson the Searcher, who once used the Power Stone to destroy an entire planet. Knowhere, a location created from the head of a dead Celestial, appears in Thor: The Dark World, Guardians of the Galaxy, and Infinity War.
Thus far, Ego has been the deadliest of the known MCU Celestials. He believed that he was the last Celestial in the galaxy, so he formulated a plan to conquer the known universe and recreate it as an expansion of his own consciousness by using human and Celestial DNA.
The Eternals, which will span thousands of years, could delve deeper into the cosmic backstory of the Celestials. We may see teh Celestials create both the Eternals and the Deviants before leaving them to their own devices.
Like Thanos, the Celestials don’t have to be a major presence in the MCU to become the primary villains for Avengers 5. Ego stated that he was the last of his kind, but that can’t possibly be true. After all, there are more than 30 of them in total and no one knows what may have happened to them.
In that vein, it’s possible that Avengers 5 may adapt the Dark Celestials storyline from the comics. Billions of years ago, a Celestial known as the Progenitor came into contact with powerful space parasites, collectively known as the Horde. After becoming infected, Progenitor crash landed on Earth and died. But his blood, vomit, and flesh impacted the evolutionary process of Earth and created even more of the Horde.
Long story short, Zgreb came looking for Progenitor billions of years later, only to be infected by the Horde himself. However, instead of killing him, Zgreb turned into a Dark Celestial and infected the rest of his race with space insects. The Avengers ended up battling the Dark Celestials, who had become rabid and uncontrollable.
While the Avengers finally defeat the Dark Celestials, it took every last one of their strength and intelligence to do so. She-Hulk, Thor, Captain Marvel, Loki, and Black Panther are all involved. Knowing how powerful and terrifying the Dark Celestials are, their becoming the primary villains in Avengers 5 would be the best way to include the Eternals in the fight. What else would bring back Earth’s defenders if not for their original creators?
Ultimately, the MCU has already laid the groundwork for the Celestials’ appearance in The Eternals and their potentially looming threat could set the stage for an iconic showdown in Avengers 5.