Character relationships are also a big trope. There could be friendships forming between characters from different universes. Maybe a character from Blue Exorcist becomes friends with a RWBY character and they learn from each other's cultures. Romance is also a common trope. It could be a love story between a character from Blue Exorcist and one from RWBY, which often involves them overcoming the differences in their worlds and abilities.
One popular trope is the crossover of characters' abilities. For example, a character from Blue Exorcist might use their exorcism powers in the RWBY world, or a RWBY character could learn about the demon - fighting techniques. Another is the team - up trope, where characters from both series join forces to face a common enemy. This could be a powerful demon in the RWBY universe or some sort of Grimm - like threat in the Blue Exorcist world.
Another trope is the 'Soul Bond' trope. It's where the male reader character has some kind of deep, spiritual connection or bond with one of the Blue Exorcist characters. This bond can be the driving force behind many of the plot developments, like they are able to share powers or emotions through this bond.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfics for both Blue Exorcist and RWBY. Just use the search function and type in the relevant keywords. Another option is FanFiction.net, which also has a good number of these crossover or related fanfictions.
One common trope is the 'Fish out of Water' where the main character, often a self - insert or an original character, finds themselves in the RWBY universe and attracts the attention of multiple characters from the show, forming a harem. For example, a character from our world suddenly ends up at Beacon Academy and has to adapt while dealing with the romantic interests of Ruby, Weiss, Blake, and Yang.
One popular theme is the exploration of the characters' relationships. For example, the bond between Rin and Yukio can be delved into deeper in fanfiction. Another theme is the expansion of the exorcist world, like creating new types of demons or exorcist techniques. Also, the theme of self - discovery for the characters is quite common, as they grow and develop in the fan - created stories.
Well, power - ups and leveling up are common themes. Just like in many games, the characters in Blue Exorcist gamer fanfiction often strive to get stronger in the game world. It adds an exciting element as they battle new and more powerful enemies. Also, exploration of unknown game areas is a theme. It allows for discovery of new items or secrets related to the Blue Exorcist lore.
A popular trope is the pet being a symbol of the Faunus' identity. It might have characteristics that are related to the Faunus' animal trait. For instance, if the Faunus has cat - like features, their pet could be a small feline - like creature. This connection between the Faunus and their pet helps to develop the character's backstory and personality in the fanfiction.
One popular trope is the 'Redemption of the Humanoid Grimm'. In many fanfictions, a humanoid Grimm might start to question its nature and purpose. Another is the 'Forbidden Love' trope, like a human falling in love with a humanoid Grimm, which creates a lot of conflict and drama. Also, the 'Power Struggle' trope is common, where the humanoid Grimm is either fighting against other Grimm or trying to gain power among humans.
The team - up trope is also quite common. Kratos joining forces with the RWBY team to take on a powerful foe. This allows for interesting battles and strategies as they combine their different fighting styles. Also, the 'lost in a new world' trope is popular. A character from either universe finding themselves alone in the other universe and having to figure out how to survive and get back home.
The idea of the triplets having to face a powerful and mysterious enemy together is also quite common. It adds an element of adventure and danger. And of course, there are themes related to their family background, like how their heritage as the sons of Satan affects their lives and relationships within the exorcist community.
Power - up and training arcs are quite common too. In these fanfictions, characters from either or both series might go through intense training to gain new powers or improve their existing ones. For instance, the exorcists in Blue Exorcist could learn some vampire - related skills from the characters in Rosario Vampire in a fanfic, or vice versa. This allows the writers to explore different aspects of the characters' abilities and personalities in a new and exciting way.