The key elements include a believable backstory for the OC. If the OC has some dark secrets or a tragic past, it can add depth to the story. In terms of the Justice League, their values and goals should be respected in the fanfiction. The OC should have a reason for joining the League, whether it's for redemption, power, or a sense of justice. Additionally, the dialogue in the fanfiction should be in - character for both the OC and the Justice League members. This helps to create a more immersive and believable story.
One important thing is the power balance. Your OC can't be too overpowered that it makes the Justice League members seem useless, nor too weak that they are just a liability. The relationships between the OC and the existing characters like Batman, Superman, and Wonder Woman need to be developed. For instance, Batman might be initially distrustful of the OC, while Superman could be more open - minded. The fanfiction should also have a good pacing, not too rushed when the OC joins and not too slow in the build - up of the story.
Character design of the OC is crucial. Their appearance, abilities, and motives should be well - thought - out. Also, the reaction of the Justice League members to the OC is important. Will they be welcoming or suspicious? Another key element is the overall plot. It should be engaging and make sense within the Justice League's universe.