Madara Uchiha would be an excellent character to include. His long - standing relationship with Hashirama, filled with both friendship and enmity, and his connection to Sasuke and the overall ninja world's history would make for a very engaging fanfiction.
Tobirama should be included. As Hashirama's brother, he has a complex relationship with Hashirama. In a fanfic, his interactions with Naruto could be really interesting. Maybe Naruto could try to bridge the gap between Tobirama's old - fashioned views and the more modern ways of the village in Naruto's time.
Another great author could be Ryu. Ryu has a knack for creating unique plotlines that involve Hashirama and Naruto. They often incorporate elements from different arcs of the Naruto series in their fanfictions, which gives a sense of familiarity yet novelty. Their stories usually have great character development for both Hashirama and Naruto.
For a more unique take, including Jiraiya could be great. His pervy but knowledgeable personality could lead to some humorous and yet useful moments. He might have some ancient knowledge about the connection between the Naruto world and the Pokémon world where Darkrai comes from. On the Pokémon side, Cynthia could be a great addition. As one of the strongest Pokémon trainers, she could offer a different perspective on Darkrai and also help in battles against any enemies that might be using Darkrai's power for evil.
Haku should definitely be included. Since Haku was Zabuza's loyal follower, his presence can add depth to the story. He could act as a mediator between Naruto and Zabuza at times. Also, Kakashi can be a great addition. His history with Zabuza makes for an interesting dynamic, and his mentoring of Naruto can play a role in how Naruto interacts with Zabuza.
Well, in addition to Naruto and Urahara, Ichigo from Bleach could be a great addition. Since Urahara has a connection with Ichigo, and Naruto's energetic and positive nature could create an interesting dynamic with Ichigo's more brooding personality.
Kakashi could be a good character to put in. His experience and wide range of ninjutsu could be useful in the story. For Thalia's side, Luke could be included. His conflicted nature and his skills would add a layer of intrigue, especially when interacting with Naruto and Thalia, as they try to figure out whether to trust him or not.
Definitely Naruto himself. His determination and positive attitude can bring a lot to the story. Lala too, of course, with her fun - loving and inventive nature. Sakura could also be a great addition. She could interact with Lala in interesting ways, perhaps learning about Lala's technology and applying some of her medical skills to help Lala if she gets injured.
Naruto himself is a must - he's the main character after all. Sasuke could also be great as his rival. Their interactions around the kit would be interesting, like if they both wanted it for different reasons. Sakura could be included too, maybe she helps Naruto figure out how to use the kit.
Definitely Sasuke. His complex relationship with Naruto provides a lot of material for fanfiction. You can explore their friendship, rivalry, and how they grow together. Kakashi is also a great character to include. His wisdom and unique jutsu can add depth to the story. And of course, Sakura. Her growth as a kunoichi and her relationships with Naruto and Sasuke can be interesting to write about.
Definitely Naruto himself. His unique abilities and his never - give - up attitude would add a lot to the story. For Salem's side, Salem herself is a must - include as she is the central figure in her world. Also, characters like Sasuke from Naruto could be interesting as his complex personality and powerful skills would create interesting dynamics. And maybe some of Salem's loyal followers like Cinder could also be included to add more conflict.