A good Itachi Uchiha x OC fanfiction often has a well - developed OC. The character should have a distinct personality, backstory, and goals. For example, an OC with a strong moral code that clashes or complements Itachi's own values can create interesting dynamics. Also, staying true to Itachi's character from the original Naruto series is crucial. If he's written out of character, it can ruin the story.
The relationship development between Itachi and the OC is key. It should be gradual and believable. In a great fanfic, we might see them start as strangers, then slowly become friends, and eventually develop deeper feelings. This process should be filled with obstacles and moments of growth for both characters. Additionally, the plot should be engaging. It could involve the Naruto world's politics, battles, or other elements that tie in with their relationship.
Another important aspect is the world - building. Since it's set in the Naruto universe, the fanfiction should accurately depict the clans, jutsus, and the overall atmosphere. If the author can create new elements that fit well within this world while also exploring the Itachi x OC relationship, it makes for a great fanfiction. For instance, the OC could have a unique jutsu that affects their relationship with Itachi in some way.