Well, one concept could be the juxtaposition of the dangerous or sharp nature of a knife with the communicative and often performative aspect of a microphone. Maybe it involves a character using the microphone as a weapon in a metaphorical sense, like using words as powerfully as a knife.
Characters can be developed based on their relationship with the knife and the microphone. If a character is more drawn to the knife, they could be shown as more of a fighter or a protector. If it's the microphone, they might be a storyteller or an advocate.
Well, a common concept in Megaman X fanfiction is the discovery of new powers for the characters. X might find a hidden ability that changes the balance of power. Another is the exploration of the Maverick virus in more depth. Maybe a fanfiction shows how it was created or how it can be truly eradicated. And of course, there are those fanfictions that rewrite the endings of the original games, giving a different outcome for the characters and the world of Megaman X.
One interesting concept could be the characters from Naruto using Yugioh cards in their battles. For example, Naruto might use a card that represents his Rasengan to gain an advantage in a fight. Another concept could be the crossovers of the different power systems. The chakra in Naruto and the magic in Yugioh could be combined in unique ways in fanfiction.
One interesting concept could be the fusion of Pokemon powers with the energy - based attacks in DBZ. For example, Pikachu might be able to use its electric attacks in a way similar to Goku's Kamehameha, but with an electric twist. Another concept could be the Pokemon trainers having to face off against DBZ villains, and they'd have to strategize using their Pokemon in a completely different battle environment. Also, the idea of Pokemon evolving in a way that's influenced by the ki energy from DBZ, like evolving into a more powerful form when exposed to a certain level of ki.
The idea of time travel being a form of punishment or a test is also seen. In some fanfictions, a character is sent back in time against their will as a way to make them face their past mistakes or to prove their worth. It adds a layer of drama to the story. And then there's the concept of time travel devices having their own personalities or rules. For example, a time - traveling watch might only work under certain conditions, like when the wearer is feeling a strong emotion, which can lead to some really interesting plot twists.
One interesting concept could be unique character interactions. For example, how the characters from 'bfdi' interact in new and unexpected ways in the fanfiction. Maybe they form alliances or rivalries that are different from the original series.
One interesting concept could be the exploration of Naruto and Kyuubi's relationship beyond the typical master - spirit relationship. For example, in some fanfictions, they might develop a true friendship where they share their thoughts and feelings more openly. Another concept could be the power - sharing aspect. Naruto might gain new and unique abilities by having a deeper connection with Kyuubi. Also, some fanfictions could focus on how Kyuubi's past and its own emotions play into the story, like its regrets or its hopes for the future.
In Naruto x NCIS fanfiction, a cool concept could be a crossover mission. Imagine the NCIS team having to deal with a threat that has ties to the ninja world. The NCIS agents' forensic skills combined with the ninjas' combat and stealth abilities would make for an exciting story. For instance, Zoro from NCIS could learn some basic ninja moves from Naruto to enhance his own combat skills in a particular case.
Well, in Naruto x Nebula fanfiction, a cool idea might be that the chakra system in Naruto has some connection to the energy within a Nebula. Maybe the Sage of Six Paths' power was somehow influenced by Nebula energy in the past. Also, there could be a story where Naruto and his friends have to protect the Ninja World from a threat that comes from a Nebula, like an alien race with powers based on Nebula forces.
One interesting concept could be exploring the emotional aftermath for the characters. For example, how Sonic deals with the consequences of his adventures on a deeper, more personal level. Maybe he has nightmares about the battles he's been in.