A possible plot for a Villainous Penumbra x Sunblast fanfiction could be that there is a prophecy about the two of them. The prophecy states that either they will destroy the world together or save it. Both are skeptical at first. Penumbra, being the villainous one, is more inclined to the destructive side. However, as they interact more, Sunblast's positive influence starts to make Penumbra question his ways. Along the way, they face many challenges that test their relationship and their beliefs about the prophecy.
To develop Penumbra, focus on his villainous side. Maybe show his past that led him to be this way. For example, if he was betrayed in the past, it could explain his current actions. For Sunblast, show his heroic nature through his actions, like saving innocent people. Then, when they interact, have Penumbra be sarcastic and dismissive at first, while Sunblast is patient and tries to understand him.
Well, you could have a villainous oc fanfiction where the oc is an anti - hero type of villain. They might do bad things but for a good cause in their own warped view. For instance, stealing from the rich to give to the poor but using very unethical methods. Or perhaps an oc villain who is a powerful sorcerer that wants to reshape the world according to their own dark desires. They could be constantly in conflict with a group of young and inexperienced heroes.
Well, you could write a fanfiction where the villains from different eras of the Harry Potter world team up. For example, Grindelwald and Voldemort joining forces in an alternate timeline. It would be really interesting to see how their different styles of dark magic would combine and how the heroes would deal with it. You could also focus on a single minor villain like Pius Thicknesse and expand on his character, perhaps giving him a more complex motivation for joining the Death Eaters than what was shown in the books.
A common 'villainous crossover fanfic' idea is to have the main villains from different fantasy series cross paths. For instance, Sauron from 'The Lord of the Rings' and Voldemort from 'Harry Potter'. They could exchange knowledge of dark magic and try to conquer both their respective worlds. Or, in a more comical vein, villains from animated shows such as Plankton from 'SpongeBob SquarePants' and Mojo Jojo from 'The Powerpuff Girls' could cross over. Plankton might try to steal Mojo Jojo's inventions to finally take over the Krusty Krab.
It's about a man who works at a bookstore and discovers a strange, coded book. This leads him on a journey into a secret society related to the book and the world of ancient knowledge and modern technology.
A third plot idea might be that Saruhiko has some sort of hidden power or secret that he's afraid to reveal to Misaki. Misaki, on the other hand, senses that Saruhiko is hiding something. As the story progresses, Saruhiko's secret starts to surface, and Misaki has to decide whether he can accept Saruhiko with this new knowledge. It would be a story full of mystery, suspense, and relationship - building.
One plot idea could be that Tommyinnit and Timedeo team up to compete in a big gaming tournament. They have to overcome various challenges together, like difficult opponents and glitches in the game.
One plot idea could be that Lucifer and Sabrina team up to solve a mystery that threatens both their worlds. They start off as reluctant allies but as they work together, they begin to understand and respect each other.
Maybe X - 23 is on the run from some anti - mutant forces and finds herself in Gotham. Batman initially suspects her but then realizes she's being hunted unjustly and decides to help her. They then have to fight off the hunters while also dealing with Gotham's regular villains.
One plot idea could be that Huntress is on a mission and meets Question by chance. They initially clash due to their different methods but gradually come to respect each other. Another could be a story set in a post - apocalyptic world where they have to team up to find a cure or a hidden treasure. A third idea is that Huntress is framed for a crime and Question helps her clear her name while they both uncover a larger conspiracy.