Well, it could be that Naruto somehow enters the world of Deadman. Maybe he has to adapt to their rules and powers. He might use his ninja skills in unique ways there.
One idea could be a crossover where Naruto characters are trapped in Deadman Wonderland. For example, Naruto could use his shadow clones to deal with the various challenges and enemies in the prison. Another idea is to explore how the powers in Deadman Wonderland would be used by Naruto characters. Maybe Sasuke could master some of the blood powers in a unique way. And a third idea could be a story where the main Naruto characters have to form alliances with the Deadman Wonderland inmates to escape the prison together.
Another plot could involve the villains from both worlds. Orochimaru from Naruto might be interested in the strange powers and experiments in Deadman Wonderland. He could form an alliance with some of the bad guys there. Meanwhile, the heroes from both universes have to come together to stop this dangerous union. They would have to combine their different fighting styles and strategies to face this threat.
Begin by choosing a main character from either Naruto or Deadman Wonderland. If you pick Naruto, consider how his ninja skills and values would interact with the harsh reality of Deadman Wonderland. You could start by creating a scenario where Naruto is thrown into the prison without understanding what's going on. Describe his confusion and then how he starts to adapt. Maybe he first tries to use his talk - no - jutsu on the guards, which of course doesn't work as expected. This would set the stage for the story to unfold. Also, don't forget to plan out the overall arc of your fanfiction, whether it's about survival, escape, or something else.
Naruto, on the other hand, would approach it with his usual optimism. He would try to make friends with as many people as possible. He would see the oppressed in Deadman Wonderland as people he needs to protect. He would use his Rasengan and other jutsus in creative ways during the battles there. He might also try to bring some of the Naruto - world values like friendship and loyalty to the rather cut - throat world of Deadman Wonderland.
I think a great Senji x Toto fanfic could involve their adventures within Deadman Wonderland. Maybe they team up to face a new and powerful enemy in the prison. Senji's tough - guy attitude and Toto's unique abilities could make for an interesting dynamic. Writers might also delve into their backstories and how they influence their relationship in the fanfic.
In a fanfiction like this, the gun could be a symbol of the danger that the Justice League, especially Batman, constantly face. Deadman, being a supernatural element, might be trying to communicate something about the gun's origin or the person wielding it. Batman, with his detective skills, would start investigating while the rest of the Justice League deals with the immediate threat of the gun - related situation.
There are many great fanfictions out there about Senji and Toto. Some might focus on their relationship development within the harsh environment of Deadman Wonderland. Maybe they explore how they first met and gradually came to trust each other.
Yes, there are likely some out there. You can check fanfiction websites like Archive of Our Own or FanFiction.net. Just search for 'Justice League Batman Gun Deadman' and you might find some interesting stories.