Often in fanfiction, Ego's actions towards Peter Quill are harmful. His grand plans might involve sacrificing Peter in some way. He could have lured Peter to his planet with false promises of a family and a great destiny, only to reveal that Peter was just a tool for Ego to consume more planets and gain more power.
Another way is through isolation. Ego could isolate Peter from his friends and allies on his planet. This makes Peter feel alone and more dependent on Ego, which can be a form of psychological harm.
Well, in this fanfiction scenario, Gamora's assassin skills would be put to great use. She could be training Peter Quill, who has his own natural talents from his space - faring adventures. Ego might be a target of the assassins for some reason, perhaps because of his god - like powers. Peter and Gamora then have to protect Ego while also dealing with their own relationship issues and the challenges of the assassin underworld.
In fanfiction, their relationship could be developed through shared experiences in the present. For example, they could be trapped together in a strange dimension. During this time, they are forced to rely on each other, talk about their feelings, and gradually build a more positive connection. Peter might learn about Ego's loneliness throughout the eons, which could make him more empathetic towards his father. This would be a big step in developing their relationship in a new way. Also, Ego could show his love for Peter in more selfless ways during this time, instead of just trying to use him for his grand plan.
One idea could be exploring what would have happened if Peter had fully embraced Ego's plan at first but then had a change of heart earlier. Another could be a story where Ego somehow survives and tries to win Peter back through different means. Maybe a fanfiction where Peter has to team up with Ego again to face a new common enemy that is even more powerful than Thanos.
Peter Quill could initially be distrustful of Ego, given his past experiences. But Gamora, being more level - headed, might see the potential in Ego's power to help them against the assassins. So, she could try to convince Peter to work with Ego.