Well, first, a good understanding of both Alduin's character from its original source and the mechanics of the Soul Eater world. For example, if the fanfic incorporates Alduin's ability to destroy worlds and relates it to the Soul Eater's concept of devouring souls in a logical way. Another key element is unique plot twists. It should not just be a simple mash - up but something that surprises the reader, like Alduin having a connection to a hidden power in the Soul Eater world.
I'm not sure of a lot of them, but there's a fanfic that focuses on the battle between Alduin and the Soul Eater characters. It's called 'Clash of Titans'. It has some amazing action scenes and unique interpretations of how the two different universes' powers interact.
One possible fanfic could be about Crona's journey of self - discovery. In the world of Soul Eater, Crona has always been an outcast. The fanfic could explore how, as a girl, she tries to break free from the chains of her past and the control of her mother. Maybe she starts to make real friends at the DWMA, like Maka and Soul, and they help her to see her own worth.
One unique feature is the integration of gaming mechanics. For example, characters might have experience points or leveling systems. Another is the use of in - game items or abilities that are translated into the Soul Eater universe. This can lead to very creative power - ups and strategies.
One common theme is the exploration of power. In both Soul Eater and RWBY, power is a central aspect. Fanfics often focus on how characters from different universes compare or combine their powers.
One great Tsugumi x Akane fanfic could be 'Tsugumi and Akane's Unlikely Bond'. It focuses on how their different personalities draw them together in unexpected ways. They start off as just classmates in the Soul Eater Not world, but gradually develop a deep friendship that might turn into something more.
One idea could be exploring the backstories of the lesser - known characters in the Soul Eater world. For example, the story of how some of the minor meisters and weapons got to the Death City. Another idea is a 'what - if' scenario, like what if Maka and Soul never became partners? You could also create a fanfic where the characters have to face a new type of enemy that is not related to kishin eggs but something completely different, like an alien invasion of the Death City.
You can start by looking on fanfic websites like Archive of Our Own. There are often many well - written Crona x Blair fanfics there. Just search for 'Soul Eater Crona x Blair' in the search bar.
In a Crona x Blair fanfic, the story might center around the power dynamics. Blair, being a powerful cat - witch, could play a role in helping Crona control their own power. For example, she might teach Crona some magic techniques that are different from what they've learned at the DWMA. This could lead to a deeper connection between them as they train together.