Well, in some Twilight fanfiction involving a mermaid Bella, she is often adapted to have unique abilities. For example, her mermaid nature might give her enhanced swimming skills that are far beyond normal vampires or humans in the Twilight universe. This could also lead to new plotlines where she has to hide her true form from the other characters.
One common theme is the exploration of Bella's identity. Since she's a mermaid in these fanfictions, it often looks at how she reconciles her new mermaid self with her human side. Another theme is the relationship changes. Her being a mermaid can greatly impact her relationships with Edward, Jacob and others.
Perhaps the Volturi could still be the main antagonists. Even though Bella is now a mermaid, the Volturi's strict laws about the supernatural world might still put her in danger. They could see her mermaid nature as a threat to their control.
Well, one interesting aspect could be the unique combination of the mermaid element with the Twilight universe. It might bring a fresh perspective to the Bella character, perhaps giving her new powers and abilities as a mermaid that could change her relationships within the Twilight story. For example, her interactions with the vampires and werewolves could be completely different.
One common plot twist is that Bella's mermaid identity is revealed in a very unexpected way. Maybe during a swim with the Cullens, she suddenly transforms, shocking everyone. Another is that her mermaid powers are somehow connected to the Volturi's plans, adding a new layer of danger.
One popular 'bella mermaid twilight fanfiction' is 'Bella the Mermaid Princess in Twilight'. It gives a really interesting take on Bella as a mermaid in the Twilight universe. The author does a great job of mixing the mermaid lore with the existing Twilight elements.
One of the most intriguing mermaid characters could be a mermaid who has a past connection to the vampire or werewolf clans. Maybe she was involved in some ancient pact or feud. Another interesting character could be a mermaid with a split personality, one side being very much in tune with her mermaid nature and the other trying to adapt to the Twilight society. And there are mermaids who are outcasts among their own kind but find a new family in the Twilight world.
Love and relationships are also prevalent. For example, in 'Bella and the Mermaid Prince', there's a romantic relationship between Bella and a mermaid prince, which adds a lot of drama and emotional depth to the story. And sometimes there are also complex friendships formed within the mermaid community in different fanfictions.
There could be a friend of Bella's, like a human friend who starts to suspect that something is different about Bella. This character could add an interesting dynamic as they try to figure out Bella's secret and support her at the same time.
Yes, there are some out there. You can start by looking on popular fanfiction websites like Fanfiction.net or Archive of Our Own. Just search for 'Twilight Bella is a werecat' and you'll likely find a variety of stories with different takes on the concept.