A more complex plotline could involve time - travel. Pyro's sister could travel back in time to prevent Pyro from making certain bad decisions that led him down a dark path in the X - Men universe. This would involve her interacting with different versions of the X - Men characters and changing the course of events.
You can try searching on fanfiction websites like Archive of Our Own (AO3). It has a large collection of X - Men fanfics. Just type in 'X Men Pyro Sister' in the search bar, and it will show relevant stories.
In a 'superman in x - men harem fanfic' plotline, Superman could be on a mission that intersects with the X - Men's world. His presence disrupts the normal dynamics among the X - Men, especially among the female members. They vie for his attention, and he has to balance his newfound relationships while also helping the X - Men with their long - standing issues like mutant - human relations. There could also be a love triangle or multiple love triangles among the X - Men female characters and Superman, adding more drama to the story.
Perhaps it could be a story where Seventh Sister has a change of heart. She starts to question her loyalty to the Empire after encountering Luke. Luke, in turn, tries to convince her to turn to the light side. They go on a journey together, and along the way, they face various challenges that test their new - found alliance.
A TF2 Pyro x Engineer fanfic might involve a mystery on the TF2 battlefield. Strange things are happening, and the Pyro and Engineer are the ones to figure it out. The Pyro's instincts and the Engineer's logical thinking combine to solve the mystery. They might discover a hidden enemy plot or a long - lost TF2 relic during their investigation.
One possible plotline could be that they meet during some sort of inter - galactic event. Maybe Gohan is there to protect Earth from a threat that also involves Blackfire's planet, and they end up having to work together. Along the way, they start to develop feelings for each other.
A plotline could involve Ayane being a mysterious figure who has some unique power. Naruto is intrigued by her and tries to get to know her better. As they interact more, they find themselves falling in love. Maybe there's also a villain who wants to use Ayane's power for evil, and Naruto has to protect her.
A plotline could involve a magical artifact that has the power to change the fabric of different universes. Naruto, Mirajane, and Erza are all drawn to it for different reasons. Naruto wants to use it to protect his village, Mirajane to protect her guild, and Erza for the sake of justice. As they fight over the artifact at first, they realize that a greater evil is trying to use it for destruction, so they join forces. Along the way, they learn about each other's worlds and cultures, and their relationships grow.
One possible plotline could be that Madara, Hinata and Sasuke are forced to team up for a mission. Along the way, they start to understand each other better and develop unexpected bonds. Maybe Hinata's kind nature starts to soften Madara's hardened heart, and Sasuke is caught in the middle of this new dynamic.
One common theme is Pyro's struggle with his powers. In many fanfictions, he has to deal with the fear of losing control over his pyrokinesis, which can be a source of internal conflict. Another theme is his relationships within the X - Men team. Some stories focus on his friendships or rivalries with other members like Wolverine or Storm.
The fanfic could also explore her relationships outside the family. She might fall in love with someone who is not part of the Sohma world. This would bring a whole new set of challenges as she has to balance her love for this person and her loyalty to her family. There could be a lot of drama as she tries to keep her relationship a secret or convince her family to accept it.