A common theme is power struggle. When servamp characters cross over with another universe, their powers often clash or complement those of the other characters. For example, if crossed with a post - apocalyptic world, they might struggle for dominance.
I really like the ones that focus on character interactions. Imagine Lambo from 'KATEKYO HITMAN REBORN' meeting Kuro from 'SERVAMP'. Lambo's childish and unpredictable nature combined with Kuro's laid - back but protective attitude towards Mahiru could lead to some really hilarious and heartwarming scenes. These crossovers often bring out new sides of the characters that we don't usually see in their original series.
One great place to find servamp crossover fanfiction is Archive of Our Own (AO3). Just search for'servamp crossover' on their site, and you'll get a bunch of results. Some popular crossovers might include ones with other popular anime series like 'Kuroshitsuji' or 'Tokyo Ghoul'.
One great 'servamp crossover fanfic' is 'Servamp and Black Butler: A Dark Alliance'. It combines the unique vampire lore of Servamp with the Victorian setting and complex characters of Black Butler in an exciting way. Another interesting one could be 'Servamp meets Fairy Tail: Magical Encounters' which brings the Servamp vampires into the world of Fairy Tail's magic and guilds.
The combination of different powers and concepts. In 'KATEKYO HITMAN REBORN', there are the unique Dying Will Flames and various combat techniques. In 'SERVAMP', we have the vampire - servant relationships and their distinct abilities. When these are combined in a crossover, it creates a whole new world of possibilities. For instance, seeing how Reborn's combat expertise would fare against a Servamp in a fight is really exciting.
In fanfiction, female Mahiru often shows great courage. She is not afraid to stand up to the powerful Servamps and other dangerous foes. For instance, in many stories, she'll bravely face a Servamp who is causing trouble in the city. She also has a strong sense of justice. If she sees something wrong, she'll do whatever it takes to make it right.
In many Servamp fanfictions where Mahiru is kidnapped, it often leads to a series of intense and emotional events. His friends in the Servamp world would surely be in a panic. Kuro, for example, would likely be extremely angry and determined to rescue him. He would use his powers to track Mahiru down. Maybe he'd team up with other Servamps or their Eves to form a search party. The kidnappers could be some old enemies who want to use Mahiru as a bargaining chip against the Servamps.
Another fanfiction worth checking out is 'In the Shadows of Lawless and Licht'. This one focuses more on the emotional side of their relationship. It shows how Licht's past experiences influence his interactions with Lawless, and how Lawless, with his carefree nature, gradually breaks down Licht's barriers. The story is filled with tender moments and emotional depth.
First, you need to decide which two (or more) universes you want to cross over. For example, if it's Servamp and Harry Potter, study the lore of both. Know the rules about vampires in Servamp and magic in Harry Potter. Then, think of a unique plot that can bring the characters together in an interesting way, like a dark magic that affects both worlds and forces them to interact.