One idea could be that the twin sister has a different combat style. For example, while Weiss uses her Glyphs mainly for ice attacks and movement, the twin sister could use Glyphs for illusions. Maybe she was separated from Weiss at a young age due to some family drama or a Grimm attack. As the story progresses, they cross paths during a major battle against Salem's forces and initially don't recognize each other. But then, through some shared memories or a family heirloom, they figure out their relation and team up to defeat the common enemy.
Perhaps the twin sister has a special ability related to summoning, but in a very different way from Weiss. Instead of summoning the usual creatures, she can summon spirits of past Schnee family members for guidance. In the fanfiction, she could be living a secret life away from the Schnee Manor, trying to find a way to free the family from its dark past. When Weiss discovers her existence, she is initially skeptical but then realizes they need to work together. They then go on a journey to discover more about their family's hidden powers and use it to protect Remnant.
The twin sister could be a pacifist in contrast to Weiss's more combat - ready nature. She might use her Glyphs not for fighting but for communication with Grimm, trying to understand their nature and find a non - violent way to coexist. Weiss might think her sister is crazy at first. However, as the story unfolds, Weiss realizes that her sister's approach has some merit when they face a situation where brute force alone can't solve the problem. Together, they try to change the way Remnant views the Grimm, while also dealing with the internal family issues within the Schnee family.
She would definitely have an impact on Weiss's relationship with Ruby. Ruby would be curious about the new addition and might try to befriend the twin sister. This could cause some jealousy in Weiss at first as she feels Ruby's attention is being divided. But over time, they would all form a stronger bond. With Blake and Yang, the twin sister could bring new perspectives. For example, she might have different views on the Faunus situation which could either create some tension or lead to better understanding among them.
One great Weiss twin fanfiction is 'The Twin Bond'. It delves into the relationship between Weiss and her twin in a very emotional way. The author does an amazing job of exploring how they interact, their shared past, and how they face challenges together. It also has some really cool fight scenes that are well - written and fit into the RWBY universe.
One possible plot could be that Weiss, seeking to gain an edge in combat against a new and powerful enemy, decides to seek out Raven for training. Raven, initially reluctant, sees potential in Weiss and agrees. Throughout the training, Weiss struggles to adapt to Raven's unorthodox methods.
One possible plot could be that Naruto's twin sister has a unique and powerful ninja ability that she has to keep hidden from the village at first. Maybe she can communicate with animals in a very special way, like having full conversations and understanding their emotions. As the story progresses, she uses this ability to help Naruto and their friends in various missions, for example, finding a lost ninja by asking the birds for directions.
First, you need to really understand Weiss's character. Know her strengths, weaknesses, and her backstory in the RWBY world. Then, when creating the twin, make sure they have distinct but related characteristics. For example, if Weiss is more reserved, the twin could be more outgoing but still share the same sense of justice. Also, have a clear plot that involves both of them, like a shared quest or a mystery to solve.
A possible plot is that there is a prophecy about a special Grimm that only Pyrrha, Weiss and William can defeat. The Grimm has been causing chaos across Remnant. Pyrrha is seen as the strong warrior, Weiss as the intelligent strategist, and William as the wildcard with some unknown abilities. They journey together to find this Grimm. Along the way, they learn more about each other and their own powers. When they finally face the Grimm, they have to put all their skills and trust in each other to win. This journey could also involve them meeting other characters from RWBY, getting help or facing additional challenges from them.
One common theme is sisterly bond. The fanfictions usually explore the deep connection between the twin sisters, how they support each other through tough times. Another theme is self - discovery. The sisters often embark on a journey to find out who they really are.
A plot could be that Weiss meets a Faunus with a special power that she's never seen before. They start off on the wrong foot, but as they are forced to team up against a common enemy, like a new type of Grimm. As they fight side by side, Weiss realizes that the Faunus is not what she expected and they develop a mutual respect for each other.
Perhaps in this fanfiction, Weiss discovers a strange portal and steps through it, ending up in the Naruto world. She meets Naruto and is intrigued by his ninja abilities. They could start training together, with Weiss learning some ninja techniques and Naruto learning about Dust. Their combined skills could then be used to protect Konoha from a new threat that has some similarities to the Grimm from her world.
Well, Ruby's twin sister could have a different view on the world of Remnant. This could lead to the team splitting up at times, with Ruby following one path and her sister another. It would add a lot of drama and new storylines as they reunite and face their differences.