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.
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.
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.
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 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.
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 the Doctor and Jack Harkness team up to save a planet from an unknown alien threat. They use their combined knowledge and skills to outwit the aliens.
A possible plotline might be that a dark wizard is after Hermione because of her new - discovered connection to the Malfoys. Draco, feeling a sense of family loyalty despite their differences, decides to protect her. Along the way, they learn more about their family's past and the reasons behind Hermione being separated from them. It could also involve Hermione trying to reconcile her Gryffindor values with the Slytherin - like values of the Malfoy family. There might be a scene where they have to face their parents together, and the Malfoy parents have to accept Hermione as their daughter, which would be a huge shift in their pure - blood ideology.