Another interesting plot could be a love story. Picture a character from RWBY falling in love with a character from Shuumatsu no Valkyrie. This would face many challenges, such as the differences in their worlds, cultures, and the fact that they are both often in the middle of dangerous situations. But their love could be a force that brings their two worlds closer together.
It's about a battle royale - style setup where gods from different mythologies battle against humans. The Valkyries play a role in setting up these battles.
First, you need to understand the characters from both series well. Know their personalities, their backstories, and their goals. This will help you write them in a way that is true to their original selves. For example, Ruby from RWBY is very optimistic and determined, while Thor from Shuumatsu no Valkyrie is strong - willed and confident in his power.
A common theme is friendship. They often start as acquaintances or even rivals and then grow to become good friends. For example, in many stories, they learn to trust each other in battle and that trust extends into their personal relationship.
Well, one interesting plot point could be how the Valkyries' unique abilities are incorporated in the fanfiction while reading the story. Maybe they use their powers to influence the outcome of the events in the story they are reading.
A prevalent theme is the exploration of their different personalities. Thor is more of a traditional hero, while Valkyrie has her own unique set of values. Their love stories in fanfiction often deal with how they understand and complement each other. For example, Thor's strength combined with Valkyrie's cunning can make for an exciting dynamic in a story. Also, the theme of sacrifice for love might be present. They could be in a situation where one has to sacrifice something important for the other's well - being or for the greater good, and this tests their love.