Wonder Woman is another character that might be paired with Itachi. Her Amazonian warrior heritage and Itachi's shinobi skills can be compared and contrasted. Their different views on justice, with Wonder Woman coming from a more idealistic place and Itachi having a more pragmatic approach, can lead to some great character - driven storylines in fanfiction.
One interesting Itachi Justice League fanfiction could be one where Itachi uses his Sharingan abilities in unique ways within the Justice League universe. For example, his genjutsu might be used to deceive powerful villains in a way that the regular Justice League members can't. It could add a whole new layer of mystery and strategy to their battles.
Batman is often a very popular character. In fanfiction, he can be portrayed in different ways, like a more nurturing mentor figure to the Young Justice team or as a master strategist leading the Justice League in complex battles.
Sasuke is often paired with Itachi in these fanfictions. Their complex relationship from the original Naruto series can be explored in new ways in another world. For example, they might be on a mission together in a world full of unknown dangers. Another popular character to be paired with Itachi is Naruto. Their different outlooks on life and friendship can create interesting dynamics, especially when they are in a strange new world where they have to rely on each other more than ever. Shisui could also be paired with Itachi. Since they were friends in the original story, in the fanfiction, they could be reunited in another world and face challenges together, perhaps against a new and powerful enemy.
Obviously Batman himself is extremely popular. He's the main focus after all. But Superman also features prominently as his counterpart and sometimes rival. Wonder Woman is also a favorite as she often has interesting interactions with Batman, whether it's in combat or in strategic planning within the fanfiction stories.
Superboy is really popular. His struggle with his identity as a clone and his place in the world makes for great fanfiction. Writers can explore his relationships with other characters like Superman, who he has a complex connection to, and his friends in the Young Justice team.
Superboy is often a popular choice. His complex origin story, being a clone of Superman, gives a lot of room for exploration in fanfiction. Writers can delve into his identity issues and how he fits in with both the Young Justice team and in relation to the Justice League.
I think Batman and Robin (from Young Justice) would be really interesting. Batman's brooding and strategic mind against Robin's youthful enthusiasm.
Sasuke can also be a significant character in such fanfictions. His relationship with both Itachi and Naruto adds another layer of complexity. His desire for revenge against Itachi and his complex friendship with Naruto can create interesting sub - plots. He might be a source of conflict or a character who eventually comes to understand the relationship between Itachi and Naruto.
Itachi himself is obviously a very popular character. His cool demeanor and powerful abilities make him a central figure in most of these fanfictions.
Danny Phantom himself is obviously very popular as he is the main character from his own series. His ghost - fighting abilities and his dual identity make him an interesting addition to the Justice League universe.