One common theme is the clash of different cultivation systems. For example, when two xianxia worlds collide, their ways of achieving immortality or power might be very different, and this creates interesting conflicts.
Another good one is 'Crossover of the Immortal Realms'. In this fanfiction, the author manages to bring together different xianxia settings and storylines. The action scenes are exciting, and the exploration of different power hierarchies in the combined xianxia worlds is quite fascinating.
Another idea is that Greninja gets exposed to some kind of meta - human - creating substance in the DC universe and gains new powers. It could then join the Teen Titans, sharing its unique Pokémon abilities with the group. It would be interesting to see how Greninja interacts with characters like Robin, Starfire, and Cyborg, and how it uses its powers in battles against the Titans' enemies.
In this fanfiction, Skynet could find a way to connect to the Matrix. It might try to take over the Matrix and use it to control more humans. The machines in the Matrix might initially resist, but Skynet could offer them a deal to share power over humanity.
Crossover fanfiction is fan - made content that blends aspects of distinct fictional realms. It can be really exciting as it gives a chance to see superheroes from the Marvel Universe interact with characters from a classic novel like 'Pride and Prejudice'. This form of fanfiction often appeals to a wide range of fans who enjoy different types of fictional works and like to see the collision of different ideas and concepts.
A '24 crossover fanfiction' is a type of fan - created work that combines the elements of the '24' franchise with other fictional universes or characters. It allows fans to explore new storylines and interactions that wouldn't occur in the original '24' series.
One idea could be that Greninja, being a ninja - like Pokémon, somehow gets transported to the Naruto universe. It could team up with Naruto and use its water - based attacks in combination with Naruto's Rasengan. Maybe Greninja's ability to blend into the shadows could be used in stealth missions in the Hidden Leaf Village.
One idea could be a St Trinians crossover with Sherlock Holmes. The girls of St Trinians could get involved in a mystery that Sherlock is also investigating. Their unorthodox ways might both amuse and frustrate the great detective.
I like the idea of a Danny Phantom and Ben 10 crossover. Ben has the Omnitrix which gives him access to different alien forms, and Danny has his ghost powers. They could either be rivals at first, each thinking their powers are better, or they could quickly team up to face a threat that requires both their unique abilities. It would be a fun clash of two very different power - based universes.