Some fanfictions might focus on Danny's role in the afterlife as a protector. He could be protecting other spirits or even trying to protect the living from a greater threat that exists in the ghost world. There could also be a theme of identity. Danny has to re - evaluate who he is now that he's dead. Is he still the same Danny, or has death changed him fundamentally? This exploration of self can make for really interesting stories.
Authors often start by changing the rules of the Danny Phantom universe. For example, if Danny is dead, they might change how ghosts interact with the living. They could make it so that Danny has a special connection to a certain object or place that was important to him in life. This gives a unique spin to the plot. Another way is by introducing new characters. Maybe a ghost guide who helps Danny navigate the afterlife.
One popular 'Danny Phantom Danny Dies' fanfiction might be 'The Aftermath'. In this type of fanfiction, the story often explores how the other characters deal with Danny's death. For example, Sam and Tucker may go through a period of extreme grief and guilt, wondering if they could have done something to prevent it. Another could be 'A World Without Danny', where the entire Amity Park starts to change in unexpected ways due to Danny's absence, showing how much of an impact he had on the town.
Another fanfiction concept is that a character from The Walking Dead dies and becomes a ghost like in Danny Phantom's world. Danny has to help this new ghost come to terms with their situation while also dealing with the zombie apocalypse around them. This could explore themes of death, acceptance, and survival all at once.
One popular theme is Danny's struggle with his dual identity. Fans often explore how he manages to keep his Phantom and Fenton personas separate while also dealing with the challenges that come with each. Another theme is his relationships, like his friendship with Sam and Tucker which can be developed further in fanfiction. Also, his battles against new ghosts and how he discovers new powers can be a common theme.
When writing, focus on the character development. If Danny meets the survivors from The Walking Dead, how would they interact? How would Danny's view of the world change? And vice versa for the survivors. Also, don't forget about the pacing. In a fanfiction with two such different worlds, it's important to keep the story flowing smoothly. You don't want to spend too much time on just one aspect, like only on Danny's powers and not enough on the zombie - filled world of The Walking Dead.
One popular theme is the dual identity struggle. Since Danny Fenton is also Danny Phantom, fanfiction often explores how he manages to keep his two lives separate and the challenges that come with it. Another theme could be his relationships, like his friendship with Sam and Tucker in the context of his ghost - fighting alter - ego. Also, stories might focus on his growth as a hero, from a newbie ghost hunter to a more experienced and powerful phantom.
One popular Sam and Danny Phantom fanfiction could be those that explore their relationship in more depth, like how they handle supernatural threats together in different scenarios. Another might be ones that focus on their friendship growing stronger over time as they face various challenges at school and in the ghost - fighting world.
One popular female Danny Phantom fanfiction could be those that explore a female version of Danny having different relationships with the other characters. For example, a story where she has a closer friendship with Sam and Tucker in a more female - centered narrative.
There are fanfictions where Danny's ghost powers are somehow connected to the demon - hunting world of Supernatural. Maybe his powers are seen as a new type of supernatural ability that the hunters in Supernatural are trying to understand. Some fanfictions explore how Danny would react to the more gritty and dangerous world of Supernatural, and how he could use his unique abilities to survive and help.
One popular 'King Danny Phantom' fanfiction might be the ones that explore Danny's newfound powers as a 'king' in different scenarios. For example, some could focus on how he rules the Ghost Zone and deals with various ghostly politics. Another type could be about his relationships with his friends and family while having this new regal status.