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.
The ending of Deadman Wonderland manga is quite intense and somewhat unexpected. The main characters face major challenges and the final outcome leaves a lasting impression.
The ending of Deadman Wonderland manga is intense. It ties up most of the loose ends, but leaves some room for interpretation. The final scenes are both exciting and emotional.
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.