Sasuke could also be a great character to pair with Tanya. Sasuke has his own complex goals and a dark past, much like Tanya in some ways. They could form an uneasy alliance. Tanya might be intrigued by Sasuke's thirst for power and knowledge, and Sasuke could be interested in Tanya's unique abilities. Their relationship could be full of tension and mutual respect as they both pursue their own agendas in the Naruto world.
Well, you could write a fanfiction where Tanya, with her knowledge of modern warfare and magic, becomes a mentor to some of the younger ninjas in Naruto. She could teach them new ways of strategizing and fighting. Also, a plot where Tanya gets involved in the Akatsuki's plans, either trying to stop them or using them for her own gain. And there's the possibility of a story where Tanya's unique abilities are mistaken for a new and powerful ninja jutsu, causing all sorts of chaos and intrigue in the Naruto world.
A common theme is survival. Given the dangerous world Tanya is in, many fanfictions focus on how she continues to survive in various difficult situations. Another theme is power struggle, as she has to navigate through different power hierarchies.
Well, Tanya is most likely still one of the main characters, as it's a fanfiction based on 'The Saga of Tanya the Evil'. Her actions and decisions probably drive a lot of the story.
Obviously, Tanya is a key character. In the 'Saga of Evil', there could be a whole range of characters that become main characters in this crossover. It might be the leader of the forces of evil in that saga, who would then have a face - to - face with Tanya. Or it could be a character who has a special ability or a unique backstory that would make their interaction with Tanya very engaging. There could also be a character who is in between good and evil in the 'Saga of Evil' world and has to make a choice when meeting Tanya.
Well, some 'Saga of Tanya the Evil Fanfiction' might delve into the political aspects of the world she lives in more deeply. They could show how Tanya's actions influence the political landscape on a larger scale. This kind of fanfiction often requires a good understanding of the original setting and can be very thought - provoking.