You could do a crossover with 'Naruto'. Imagine the ninjas from Naruto arriving at Deadman Wonderland. Naruto's positive attitude could inspire the inmates, and his jutsu could be a powerful weapon in that world. The sharingan of Sasuke might be able to see through some of the illusions or secrets in Deadman Wonderland. Or a crossover with 'One Piece', where the Straw Hat Pirates end up in Deadman Wonderland. Luffy's rubber powers and his fearless nature could disrupt the order of the prison, and they could search for a hidden treasure within the prison walls.
One idea could be a crossover with 'Attack on Titan'. The Deadman Wonderland characters could find themselves in the world of Titans and have to use their unique abilities to survive. For example, Ganta's ability to shoot his blood bullets might be useful against the Titans. Another idea is a crossover with 'Fullmetal Alchemist'. The alchemical world could interact with the prison - like setting of Deadman Wonderland, and perhaps there could be a search for a way to break the curse in Deadman Wonderland using alchemy.
One idea could be exploring the backstories of the minor characters in Deadman Wonderland. For example, you could write about how a particular inmate ended up in the prison. Another could be a what - if scenario, like what if Ganta had different powers. Maybe he could control water instead of his blood bullets.
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.
A cool 'alice in wonderland crossover fanfiction' idea is a crossover with 'Alice in Wonderland' and 'Percy Jackson'. Alice could end up at Camp Half - Blood. The demigods would be very confused by her, and she would be equally confused by all the Greek mythology stuff. Also, a crossover with 'Sherlock Holmes' could be interesting. Sherlock could be called to Wonderland to solve the mystery of the disappearing Cheshire Cat. He would use his deductive skills in this very strange and magical place.
A plot could involve Shiro getting sick. In Deadman Wonderland, there are no proper medical facilities for inmates. Ganta has to find a way to cure her. He teams up with some of the other more friendly inmates to search for a cure. This story could show how far Ganta is willing to go for Shiro and also how the other inmates view their relationship.
First, decide which other universe you want to crossover with. Then, research both the Deadman Wonderland world and the other world thoroughly. Know the rules, the characters, and the overall atmosphere of each. Next, come up with a central plot. It could be something like a common enemy that both universes need to face together. For example, if crossing with 'Bleach', the Hollows could be attacking both the Soul Society and Deadman Wonderland. Start with a strong opening that grabs the reader's attention, like a sudden appearance of a character from one universe in the other.
Power dynamics also feature prominently. When different power systems from the two universes collide, like the Deadman's powers and the powers from another series, there is a lot of exploration about who is stronger, how the powers interact, and if they can be combined. For example, if crossing with 'My Hero Academia', the Quirks and Branches of Sin would be compared and contrasted in many ways. This could lead to battles to establish dominance or cooperation to defeat a greater threat. And it often makes for a very exciting read as the characters figure out these new power relationships.
There are also some crossovers with 'Fullmetal Alchemist'. The idea of alchemy in the world of Deadman Wonderland could lead to some interesting plot twists. Maybe the Alchemists would be intrigued by the strange powers in Deadman Wonderland and try to study them, while the Deadman Wonderland characters could be both in awe and a bit scared of the alchemical abilities.
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.
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.
Shiro is also a fan - favorite. Her mysterious past and her connection to Ganta make her a great character to write about. You could write a story that reveals more about her origin, or a story where she has to make difficult choices that put her relationship with Ganta at risk.