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.
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 trope is the mermaid's struggle to fit into the Twilight world. Since the Twilight series has its own set of rules about vampires and werewolves, a mermaid character often has to find her place among them. Another is the love triangle, which is a staple in Twilight. A mermaid might find herself in love with a vampire or a werewolf, creating a complex romantic situation. And the idea of the mermaid having some sort of special power that is unique to her mermaid nature but also relevant in the Twilight universe, like being able to communicate with sea creatures and using that to help the Twilight characters in some way.
There's a fanfic called 'Mermaid Twilight and the Enchanted Pearl'. In this story, Twilight, as a mermaid, is on a quest to find a powerful enchanted pearl to save her underwater kingdom. It's full of action and interesting characters, like her mermaid friends who each have their own unique abilities and personalities.
Love and attraction are also big themes. Just as in the original Twilight series, there are often love triangles or complex relationships. Maybe a mermaid falls in love with a vampire, but there's also a werewolf vying for her attention. It adds a new layer of drama to the story.