First, you need to be familiar with both universes. Read the Harry Potter books and watch the movies, and do the same for Deadman Wonderland. Then, decide on a central plot. It could be a crossover where characters meet, or it could be a more complex story where the worlds start to merge. Create new characters if necessary, and make sure to keep the tone consistent with both series.
A Harry Potter - Deadman Wonderland fanfiction could focus on the concept of magic vs the extreme and cruel system of Deadman Wonderland. The wizards might be seen as a threat to the order of Deadman Wonderland, and the Deadman Wonderland characters might be in awe or fear of the magic. There could also be a plot where a character has to combine the magical knowledge from Harry Potter with the survival skills needed in Deadman Wonderland to overthrow the corrupt system in 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.
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.
One great fanfiction could be 'The Unbreakable Bond'. It beautifully explores the relationship between Ganta and Shiro in a way that really stays true to their characters from 'Deadman Wonderland'. It has some really sweet moments that fans of their relationship will love.
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.