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.
One possible fanfic could be about Shiro and Ganta's adventures in the cruel world of Deadman Wonderland. It might focus on how they rely on each other for survival. Shiro's unique personality and her relationship with Ganta can be explored deeply. For example, it could start with them facing a new and dangerous game in the prison - like arena, and how they use their individual strengths to overcome the challenges together.
A possible fanfic could be a what - if scenario. What if Shiro had lost her memory completely instead of just parts of it? Ganta would have to find a way to make her remember their bond. This could involve him taking her to all the places in Deadman Wonderland that held special memories for them. It would be a story full of emotion, as Ganta struggles to bring back the Shiro he knows.
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.
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.