Well, you could write a reign fanfiction that focuses on the younger years of the main characters. For example, what was Mary like as a child in France before coming to Scotland? You could also do a story about how the political alliances in the show might have been different if certain characters had made other choices. Maybe a fanfic where a character from a different historical era somehow gets transported into the reign universe and causes chaos or helps in unexpected ways.
A great concept for 'Danny Phantom Reign Storm' fanfiction is to introduce a new character. This character could be from another dimension and have unique powers. They could either help Danny or create more chaos during the 'Reign Storm' situation. You could also write a fanfiction where Danny has to deal with the aftermath of 'Reign Storm' in a different way than in the original show, like having to repair relationships that were strained during the chaos.
Maybe in the fanfiction, Reign Bash and Kenna find a baby with special powers. They decide to raise the baby in secret while also trying to figure out the origin of these powers. As they do this, they face opposition from those who fear the baby's powers.
A possible plot idea is that the baby is born prematurely. Mary and Francis are worried about the baby's survival. They turn to their most trusted advisors and physicians, but there are also those who see this as a sign of weakness in the monarchy. Meanwhile, there could be an enemy lurking in the shadows, planning to use the baby's fragile state to their advantage. As Mary and Francis fight to protect their baby, they also have to deal with the internal politics and power struggles within the court.
One interesting story could be about the exploration of a hidden magic within the Twilight Reign world. Maybe a character discovers an ancient spell that has the power to change the balance of power among the different factions.
One interesting plotline could be Spider - Man having to operate secretly under the Dark Reign regime. He might have to find new ways to help the people of New York without getting caught by Norman Osborn's forces.
One plot idea could be exploring Hawkmoth's past more deeply. Maybe he had a tragic love story that led him to become so obsessed with the Miraculous. Another idea is to have a character from the fanfiction be a double agent, working for both Hawkmoth and the heroes. This would add a lot of suspense and mystery.
One interesting storyline could be Spider - Man having to operate in the shadows during the Dark Reign era. He might be hunted by Norman Osborn's forces while still trying to protect the city. Another could be his interactions with other heroes who are also on the run or trying to resist Osborn's regime.
One interesting butterfly reign fanfic could be about a hidden world of butterflies where they have their own monarchy. The main character, a human, accidentally stumbles into this world and has to navigate the complex politics among the butterfly reign. It's a story full of magic and discovery.
There might be a fanfic where Mary's pregnancy is a secret at first. She has to hide it from those who would oppose her having an heir. This could lead to all sorts of tense situations, like sneaking around to get proper medical care and advice without being detected.
The unique interpretations of the original Twilight characters and concepts. For example, if a fanfic gives a different backstory to Edward or a new power to the vampires, it can be really interesting.