In the Marvel Cinematic Universe, the heroes have done their best to contend with threats that range from deeply personal to genuinely cosmic -- with the stakes now reaching a multiversal level. However, the heroes could have had an unlikely and powerful ally who could have helped contend with all that -- if only his ambitions hadn't ruined the happy family life he could have had. The MCU version of Ego the Living Planet from the Guardians of the Galaxy films gave up the chance to have a normal life on Earth with a family -- and it got him killed, but if he decided to remain with Meredith Quill, he might have not only saved himself but helped establish a better version of Earth.
The backstory of Guardians of the Galaxy Vol. 2 revealed that Ego -- reinvented in the MCU as a Celestial being -- made his way across the galaxy trying to sire an heir capable of wielding cosmic power. His time on Earth led him to form a romance with Meredith, a sweet-natured woman from Missouri.
The two actually seemed to be falling in love, with Ego later admitting he came very close to staying on Earth to be with her; however, committed to his plan to spread his consciousness across all of creation, Ego ultimately decided to remove that temptation by placing a brain tumor within her -- resulting in her death. Ego's decision to condemn the woman he loved to a painful death because of his temptation is the ultimate sign to Star-Lord that his father is evil, leading to the climatic battle of the film and Ego's demise.
However, it's worth considering -- especially with the multiverse now fully opened and able to expand -- how things might have played out in another reality. It's possible that in one timeline, Ego's emotions ended up overtaking his ambitions and he decided to remain on Earth as a "human" to be with Meredith. That one small change might have been enough to radically alter the timeline in plenty of unexpected ways -- starting with Peter Quill, who would never have been in a position to be kidnaped by Yondu on Ego's orders if the latter remained on Earth. On top of sparing Peter his cosmic future, this would have kept Yondu from breaking the Ravager Code -- keeping him in the good graces of the likes of Stagard Ogord.
While the cosmic corner of the MCU might have been severely changed by this, the Earth would have likely gone through some other serious changes, especially considering Ego's place as a Celestial. This cosmic importance means that Ego remaining on Earth might have caught the attention of the Eternals, who, due to their place as servants of the cosmic beings, might have seen Ego as something of a leader. An Ego who cared about the protection of humanity and the family he'd ended up with might have been more willing to bring the Eternals out into the open, potentially allowing them to get involved when the world was threatened by alien hordes in Avengers, Dark Elves in Thor: The Dark World or robots in Avengers: Age of Ultron.
In this series of events, the Avengers and the Eternals might have become allies, leading to a far more successful series of victories for the heroes. Ego might have been able to even reveal the existence of the Celestial Egg to the heroes, giving them an opportunity to resolve the conflict without resorting to fighting against the Celestial.
Ego choosing to be a family man could have done a lot of good for the MCU, and speaks to one of the franchises' most enduring themes. Redemption is always available for heroes and villains alike -- Tony Stark, Stephen Strange and Wanda Maximoff are all characters defined by overcoming the mistakes they've made to try and do better. However, some, like Ego, can't take that chance for happiness, and they not only doom themselves, but deny the world a lot of potential advancements.