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.
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.
The Valkyries are likely to be the main characters. They are central to the concept as they are the ones doing the reading of the story.
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.
Some of the main characters are the Valkyries, and of course, the human and god participants in the battles.
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.
One idea could be a crossover where Naruto characters are transported to the Valkyrie universe. They could team up with the Valkyries to fight a common enemy, like an ancient evil that threatens both worlds. Naruto's skills in ninjutsu would be a great addition to the Valkyries' combat capabilities.
One idea could be that Oscar is kidnapped by a group of Grimm - inspired bandits. They might want to use him as a bargaining chip against the Huntsmen and Huntresses. They could hold him in an abandoned mine or a hidden fortress in the wilds.
One idea could be that Arthur Morgan somehow gets transported to the RWBY universe. He could use his skills as a gunslinger and survivalist to help the RWBY characters in their battles against Grimm. Maybe he teams up with Ruby and teaches her some of his unique combat techniques.
One idea could be a story where Ruby and Coco team up for a special mission. Maybe they have to infiltrate an enemy base, and Ruby's speed and Crescent Rose combined with Coco's fashion - inspired weapons and strategic mind make them an unbeatable duo.
One possible plot idea could be that they are in the middle of a battle, and in a moment of victory and adrenaline, they share a passionate kiss. It could be a turning point in their relationship, from comrades in arms to something more romantic.