A possible plot could be that they meet while on a dangerous mission. Maybe they are both trying to steal some important information from the same evil force. This would lead to an initial mistrust between them as they think the other might be a spy, but then they realize they have a common goal.
One plot idea could be that Scarlet has been kidnapped by the Lunars, and Cinder, who is on a rescue mission, stumbles upon her. Cinder uses her Lunar powers and technological skills to break Scarlet free. Along the way, they bond over their shared experiences of being oppressed in different ways. Scarlet learns about Cinder's true identity as the lost princess, and they form an alliance to fight against the Lunar Queen.
One great place to start looking for 'cinder meets scarlet' fanfiction is on Archive of Our Own (AO3). There are often many well - written stories there that explore different scenarios of their meeting. You can search by tags related to Cinder and Scarlet from 'The Lunar Chronicles' to narrow down the results.
One idea could be Cinder being transported to an alternate universe within the RWBY world where the roles are reversed. For example, she could find herself in a world where she is a huntress in training, facing the same challenges as the main characters but from a different perspective. Another plot could involve a team - up between Cinder and a group of RWBY characters against a common, greater enemy that threatens both their goals and the world. Maybe a new, powerful Grimm that has abilities they've never seen before.
One plot idea could be that a group of dark mages kidnap Erza to use her as a bargaining chip against her guild. They know her strength and reputation, so they think they can force Fairy Tail to give up some valuable magical artifacts. Another could be that an obsessed fan of Erza's kidnaps her, believing that she can be 'tamed' and made to love him. This could lead to a story of Erza having to outwit this deranged individual while also trying to escape from his clutches.
Another plot could be about their wedding planning. Since they are from different backgrounds, there could be cultural clashes. For example, Cinder might have some traditions from her side that Kai's people find strange, or vice versa. And they have to find a way to incorporate both cultures into their big day while also making sure their families and kingdoms are happy. It could be a really sweet and comical story as they struggle with things like choosing the right wedding dress or the proper ceremony format. Also, there could be some jealous suitors from the past who resurface and try to break them up, adding some drama to the story.
A more emotional plot could be that they discover they have a shared past that they were unaware of. Maybe they both had a connection to a certain character who was important in their lives before they became who they are now. This revelation makes them question their own paths and they start to see each other in a new light, leading to a tentative alliance and exploration of their similarities and differences.
One idea could be that the Wolf is a protector of the Scarlet Baby. The baby has some special power that many are after, and the Wolf uses its strength and instincts to keep the baby safe from all kinds of threats in a magical forest.
One idea could be that Miss Scarlet gets involved in a case that has international implications. Maybe a stolen artifact that is crucial for diplomatic relations. The Duke, of course, tries to keep her safe while she is determined to solve it on her own terms.
One idea could be a story about a trainer who discovers a rare and powerful Pokemon that has the ability to change forms in unique ways, not seen in the regular game. This Pokemon could be the key to solving a mystery in the Paldea region, like why certain areas are experiencing strange weather patterns. Another plot could focus on a group of friends who enter a secret, uncharted part of the region while on their Pokemon journey and find a new type of Pokemon that they must protect from poachers.
A cool plot would be if Scarlet Witch accidentally creates a portal that brings Naruto and his friends into her world. Naruto has to figure out how to get back home while also helping Scarlet Witch deal with some internal struggles she has with her powers. This could involve Naruto using his ninja wisdom to guide her in controlling her magic better. The story could also have them facing off against some common enemies in Scarlet Witch's universe who are after her power and see Naruto as an obstacle.
Another plot could involve a crossover with the Harry Potter world. Erza could be a transfer student to Hogwarts. Her magic, which is different from the wizarding magic, would create a stir. She might have to face off against Voldemort's forces while also learning about the magical world of wands and spells.