Peter's character could become more hardened. Killing Thanos, a powerful and feared villain, would weigh heavily on him. He might struggle with the moral implications of taking a life, even if it was to save the universe. This could lead to a more somber and introspective Peter.
One consequence could be that Peter Quill Parker becomes a hero. People all over the universe might look up to him. But he may also face some jealousy or envy from other powerful beings.
In the fanfiction, Peter Quill Parker might have some new powers or gadgets that give him the edge. He could have studied Thanos' fighting patterns and found a way to counter them. Also, his quick thinking and ability to improvise in tough situations would play a big role. He might even use his charm to get close to Thanos and then strike when he least expects it.
One possible plot could be that Peter Parker discovers a new power within himself that gives him the edge to take on Thanos. Maybe it's a power related to the Time Stone that he manages to manipulate in a way that Thanos can't counter. He could use this power to outwit Thanos in a battle and finally deliver the killing blow.
Peter could use his agility. Since he's super - agile, he can dodge Thanos' attacks easily. Then, when Thanos is off - guard, Peter can strike. For instance, he can web up Thanos' hands so he can't use the Infinity Gauntlet properly.
Peter Parker might develop by becoming more of a leader. In the fanfiction, he could step up and take charge in certain situations, guiding the other heroes. This would be a big change from his more inexperienced self in the movies.
On the other hand, Percy could become more of a revolutionary figure. His act of killing Zeus could inspire other demigods to stand up against the unjust ways of the gods, and Percy would have to step into a leadership role, which would force him to grow in terms of strategy and diplomacy.
His character could become more independent. Without some of his usual allies, he'd have to rely on himself more. He might develop new skills and strategies for fighting crime.
He'd probably become more confident. Being around such powerful heroes would boost his self - esteem.
Their relationship could start with a bit of animosity. Yelena might see Peter as a naive kid at first. But as they face dangerous situations together, she starts to see his courage and intelligence. Peter, on the other hand, is initially intimidated by Yelena's tough exterior, but then comes to appreciate her loyalty and tenacity. So, gradually, they become friends who can rely on each other.
His character often becomes more mature. By time traveling during the Civil War, he sees the consequences of the heroes' actions on a larger scale. This makes him more responsible and strategic in his decisions.