Common themes include the clash of cultures. Bella comes from a different world than the Volturi, and Felix is part of that world. So, how they deal with their cultural differences is a big theme. Survival is also important. In a dangerous place like the Volturi, both Bella and Felix have to find ways to survive. And finally, the theme of acceptance. How the Volturi accept or reject their relationship is often a key part of these fanfictions.
Often, their interactions are full of tension. Since the Volturi and the Cullens have a complex relationship, Felix and Bella are on opposite sides at first. However, in some stories, Bella's kindness towards Felix, despite his fearsome reputation, can start to change his perception of her. He may start to see her as an individual rather than just a part of the Cullen clan, leading to more positive interactions.
A great fanfiction could be set in an alternate universe where the Volturi and the Cullens are not enemies. In this story, Felix and Bella meet during a peace - making event. They find that they have a shared love for art, which is a very unexpected common ground for a Volturi guard and a human - turned - vampire. This leads to a series of interactions that slowly turn into a deep bond.
Sure. 'A New Beginning in Volterra' is a great one. It explores Bella's unexpected relationship with Felix in the Volturi world. Another is 'Bella and Felix: An Unlikely Bond' which delves deep into how their bond forms against all odds. And 'Felix's Redemption through Bella' shows how Bella's presence changes Felix in the Volturi context.
One common theme is power struggle. Since the Volturi are so powerful, fanfictions often explore how Bella either challenges or is influenced by that power. For example, she might try to assert her own will against their rules.
They might be either amused or offended. The Volturi are very traditional and strict in the Twilight universe. Fanfiction often takes creative liberties that could go against their values.
Power is a common theme. The Volturi are a powerful vampire coven, so fanfictions often explore how they use and maintain their power. For example, in many stories, they deal with threats to their authority.
One common theme is love and relationships. Since the Twilight series itself is centered around various kinds of love, fanfictions often explore how the original character (OC) fits into this world of complex relationships, especially in relation to Felix.
They might be either amused or offended. Fanfiction often takes creative liberties with the original characters and storylines. If the Volturi in the fanfiction are depicted in a way that goes against their self - perceived dignity in the original Twilight Saga, they could be offended. On the other hand, they might also find some of the more creative interpretations and new plotlines quite amusing.