A really cool plot could be that there is a threat to both the Danny Phantom universe and the Sekirei universe. Some sort of dark entity that can traverse between the two worlds. Danny and a group of Sekirei have to team up to stop this entity. The Sekirei can use their combat skills and unique powers, while Danny uses his ghost powers. Along the way, they learn from each other, with Danny teaching the Sekirei about the concept of ghosts and the Sekirei showing Danny different ways of fighting that he hadn't seen before.
Danny would be one of the main characters, no doubt. And from the Sekirei series, Musubi could be a key character in the fanfiction. She is one of the more prominent Sekirei, being kind - hearted and strong. Her naivete might lead to some humorous situations when she first meets Danny. Also, Tucker Foley from Danny Phantom could be another main character. He could use his tech - savvy skills to help in the Sekirei world, perhaps by creating some new gadgets that work in that universe as well.
One possible plot could be that Danny Phantom, while exploring the Ghost Zone, stumbles upon a portal that leads to the Transformers' universe. There, he teams up with the Autobots to fight against a new threat that is related to both ghosts and the Decepticons. For example, a Decepticon scientist could be experimenting with ghost energy to create powerful weapons, and Danny's ghost powers are needed to stop it.
A plot might be that Dora is on an adventure in a spooky forest when she accidentally crosses over into Danny's world. Danny, thinking she's a new kind of ghost at first, tries to capture her. But then they realize they are both in a strange situation. They decide to team up to find a way for Dora to get back home. During their journey, they encounter other ghosts and obstacles, and Danny has to protect Dora while she uses her wits to solve some of the puzzles they come across. Their friendship blossoms as they face these challenges together.
A plot could be that Dash discovers Danny's secret identity as Danny Phantom. At first, he's shocked and a bit scared. But then, he realizes that Danny has been using his powers for good. He decides to keep the secret and even helps Danny out in some small ways, like covering for him when he has to disappear to fight ghosts. This new - found alliance between them could grow into a real friendship over time.
In this fanfiction, it could be that Danny Phantom meets a Danny Phantom from a parallel fan - made story. At first, they don't trust each other. But as they learn more about each other's worlds, they realize that there are some common elements that are being threatened. It could be that a powerful ghost is trying to rewrite all the fanfic universes related to Danny Phantom. So, they must combine their strengths and knowledge to stop this from happening.
A possible plot is that there's a magical experiment gone wrong at Hogwarts that creates a portal to Danny Phantom's world. A female Danny Phantom enters the Harry Potter world through this portal. The Ministry of Magic thinks she's a threat at first, but she proves herself useful when a new dark force emerges. She teams up with Harry and his friends, using her intangibility and other powers to their advantage in battles and sneaking around the castle.
Well, perhaps in the fanfiction, Danny and Sam have been through a lot together, like countless battles with powerful ghosts. One day, Danny decides it's time to propose to Sam in a very special way. He takes her to their favorite haunt (pun intended), a place where they first discovered a powerful ghost artifact. There, he gets down on one knee and asks her to marry him. And of course, she says yes.
Maybe in this crossover, some Pokemon are accidentally sent to Danny's world. Danny has to protect the Pokemon from being captured by the Guys in White while also trying to find a way to send them back. Meanwhile, a certain Pokemon, like Gengar, forms a special bond with Danny because of their shared ghostly nature.
A possible plot could be that Sidney discovers a hidden power within himself that can only be activated when he teams up with Danny. They accidentally stumble upon an ancient prophecy that foretells of a great danger coming to their town. As they try to decipher the prophecy, they also have to deal with the regular high school problems like bullies and exams. In the end, they realize that their friendship and combined powers are the key to saving the town from the impending doom.
The class trip could be to a place known for its scientific research. Danny starts to notice strange energy readings that seem ghost - related. He has to work with his friends Tucker and Sam to figure out what's going on. Meanwhile, a bully from school also notices Danny acting suspiciously and tries to expose him as a 'freak', adding to the drama of the fanfiction.