Well, start with a strong concept for your Gamer OC. Maybe they have a special power related to gaming, like being able to manipulate virtual reality in the real world. In the Justice League setting, they could bond with Cyborg over technology. As for the story, it could revolve around a new threat that only the OC's unique skills can help solve. You need to keep the personalities of the Justice League members true to the source material while showing how your OC shakes things up.
For a Gamer OC x Justice League fanfiction, it's crucial to make your OC stand out. Consider giving them a quirk such as they can level - up their own abilities like in a game. When it comes to interacting with the Justice League, think of scenarios where the OC challenges the heroes' usual ways. For instance, they might suggest a more unorthodox approach to fighting a villain. You also want to build a relationship arc between the OC and the League members, whether it's friendship, rivalry, or something in between.
To write an engaging Gamer OC x Justice League fanfiction, first, develop your original character (OC). Give them a unique backstory, like they were a top - ranked gamer who got sucked into the Justice League's world. Then, think about how they'll interact with the League members. For example, your OC could use their gaming skills to strategize during battles. Also, create some conflict, like the OC's different mindset from the traditional heroes causing friction within the team.