With the MCU embracing its cosmic sector more than ever, the induction of Galactus seems inevitable, yet the perfect opportunity to introduce him already happened in Phase 3. Galactus and his herald, the Silver Surfer, need an MCU do-over and the best way to do that is by deviating from the most obvious path of bringing them in with the Fantastic Four. Not only is a shake-up essentially necessary at this point, but Galactus could be used to further expand Marvel's cosmic lore.
First appearing alongside his herald in Fantastic Four #48 (1966), Galactus, The Devourer of Worlds, is a cosmic force of nature. He feeds on the energy of whatever world that the Silver Surfer leads him to. Obviously, that creates issues when Galactus comes to Earth and his presence usually requires the intervention of the Fantastic Four, the Avengers, and whoever else Earth can spare. Galactus and the Silver Surfer made their big-screen debut in 2007's Fantastic Four: Rise of the Silver Surfer, though the film received a considerable amount of criticism for its depiction of Galactus as seemingly just a giant space cloud.
The nature of Galactus means that he has few actual consistent adversaries, but there is at least one foe who continually gives him trouble: Ego the Living Planet. Debuting in 1966 (the same year as Galactus) in Thor #132, the Living Planet has had a long and heated rivalry with the World Eater. A clash between the two titans of the cosmos would naturally be a perfect way to bring Galactus and his herald into the MCU. With the introduction of Eternals' Celestials, the MCU has shown Galactus is possible to pull off in a convincing way.
Ego was killed at the end of Guardians of the Galaxy Vol. 2, but the MCU has already made a film about a canonically deceased character, Black Widow, by making setting it earlier in the timeline. A film depicting a clash between Galactus and Ego could be handled in very much the same way. Galactus isn't likely to get his own film, but a Silver Surfer movie early into his existence could show such a battle taking place, long before the Surfer ever comes to Earth, allowing the character and his master to be established early. Then they could appear as the antagonistic force of an MCU crossover film worthy of their scope, rather than simply showing up in yet another Fantastic Four movie to rehash the celebrated comic book storyline.
Galactus and Ego's battles traditionally end in stalemates, eliminating concerns over a continuity crisis that'd result from the elephant in the room of Galactus tending to devour the planets he visits. Because Galactus and Ego's clashes have historically ended with neither side being destroyed and one simply being driven off, this would allow for Galactus to appear again in a proper MCU crossover like The Avengers. And a movie that takes place earlier in the timeline wouldn't retcon Ego's role in Guardians of the Galaxy vol. 2.
Further than that, a return for Ego and the establishment of a Silver Surfer film franchise would allow for the MCU to expand its cosmic lore. Eternals elaborated on the concept of Celestials that Guardians of the Galaxy vol. 2 introduced. However, the two film franchises seem to conflict with one another in exactly how Celestials work. A return for Ego could clear things up, hopefully using smart writing solutions as opposed to hard retcons. After all, his comic book counterpart isn't a Celestial at all, so there's quite a bit that could be done with this. Whatever the case may be, Ego the Living Planet provides a perfect starting point for the World Eater and his herald in the MCU. If Galactus does appear to make waves in the MCU, there truly is no better fit for his introduction than a battle with Ego the Living Planet.
