Well, it could be about Jane's experiences in the Naruto world. Maybe she's a new character who records her daily adventures, like her encounters with Naruto and his friends, and how she copes with the challenges of that world.
In the 'naruto diary of jane fanfiction', it might focus on Jane's perspective of the ninja world. She could be writing about the political situations she witnesses, like the conflicts between different villages. Also, she might be documenting her training in ninja techniques if she's a ninja herself. Her diary could serve as a way to show the readers a different side of the Naruto universe that we don't usually see through the main characters' eyes.
Jane is likely a fan - created character. She could be an ordinary girl who somehow gets transported into the Naruto universe and starts writing a diary about her new life there.
Maybe Naruto uses the diary to talk about his relationships. He could write about his friendship with Sasuke, how much he misses him when he's away, and his determination to bring him back. He might also write about his feelings for Sakura, like how he's trying to become someone she can rely on.
Another plot twist might be that Mary Jane discovers that the Black Cat has been working undercover for a good cause all along, but has been framed as a villain. Mary Jane then has to help clear her name. This would turn the typical perception of the Black Cat on its head.
One possibility is that Naruto's friends launch a rescue mission. They would use their unique skills, like Sasuke's Sharingan or Sakura's medical ninjutsu combined with her combat skills, to find and save Naruto.
Perhaps in a fanfiction, there is a magical anomaly that fuses Naruto's world and Mary Jane Watson's world. Naruto and Mary Jane find themselves in a strange new realm that has elements of both universes. They have to work together to find a way back home. Along the way, Naruto teaches Mary Jane some basic ninja skills like how to move stealthily and how to sense danger. Mary Jane, in turn, shows Naruto the glamour and art of the theater world. Their friendship grows as they face various challenges in this mixed - up world.
Well, a plot could involve Naruto teaching the Volturi some of his ninja skills. Jane might be a quick learner and use these skills to gain more power within the Volturi ranks. Or perhaps Naruto falls in love with Jane, which causes a stir in both the Naruto universe and the Volturi world. Their relationship could be a catalyst for a new era of peace or chaos depending on how others react to it.
Perhaps the Jester and the Dragon don't get along at first in the fanfiction. This could create a lot of interesting situations. Jane would then have to play the mediator and in the process, they all learn to work together better. There could be some humorous exchanges between the Jester and the Dragon as they slowly become friends.
A possible plot for this crossover fanfiction could be that the Future Diaries are actually some kind of ancient artifacts in the Naruto world that were lost long ago. Naruto discovers one while on a mission. The power of the diary starts to change the balance of power in the ninja world. Sasuke might be intrigued by Naruto's new power and at first be a bit jealous. But then they realize that they need to work together to stop a greater evil that is also interested in the power of the Future Diaries. The Future Diary characters would then come into play as either allies or enemies depending on their motives in this new world.
One possible plot could be that Naruto is somehow cursed or experimented on and turns into a reptile. He then has to figure out how to reverse the transformation while still dealing with the threats in the Naruto universe.
Maybe Naruto could be corrupted by some dark force. He might start using his powers for evil, like attacking Konoha instead of protecting it. He could form alliances with villains from the series and use his knowledge of the village to undermine it.