Yes, there are quite a few. Some popular ones can be found on fanfiction.net. You can search using the tags 'Bella Swan' and 'Caius Volturi' to narrow down the results. They often explore unique relationships and storylines within the Twilight universe.
Well, a frequent theme is the exploration of different vampire cultures. Caius being part of the Volturi, his relationship with Bella can show how different vampire groups interact. Also, the theme of redemption can be present. Maybe Caius is seen as a rather cruel character in the original, but in fanfiction, his relationship with Bella can lead to a path of redemption for him. And of course, there's always the theme of love defying boundaries, as their union would be quite unexpected in the context of the Twilight world.
One interesting plotline could be Bella's struggle to adapt to the Volturi lifestyle while being Caius' mate. She might face opposition from the other Volturi members, adding drama.
In many fanfictions, Bella starts to understand the complex world of the Volturi better. She learns about their laws, politics, and hierarchies. Caius, in turn, begins to see Bella not just as a mate but as an equal. He might start to value her opinions more and include her in Volturi affairs. For instance, Bella could use her knowledge of the human world to help Caius make decisions regarding vampire - human relations.
They're an interesting pairing because they are so different. Caius is from the ancient and powerful Volturi, with a strict view of vampire laws, while Bella is a human - turned - vampire with a very different moral compass. This contrast can lead to really engaging storylines in fanfiction.
Another theme is the exploration of the Volturi world through Bella's eyes. Since she was not initially part of the Volturi, her perspective can be used to show the reader more about their customs, politics, and how Caius fits into all of that. This can range from descriptions of the Volturi castle to the complex relationships between the Volturi members as Bella experiences them while getting closer to Caius.
Some fanfictions explore a scenario where Caius and Bella form an unlikely friendship. Maybe Bella is sent to the Volturi as an emissary from the Cullens and Caius, initially cold, slowly warms up to her because of her unique perspective on the vampire - human world.
One popular 'bella swan and caius volturi fanfiction' is 'The Volturi's New Interest'. It takes an interesting twist on their relationship. Bella finds herself in the middle of Volturi politics and Caius starts to have an unexpected attraction to her.
One great Caius Volturi and Bella fanfiction is 'The Volturi's Obsession'. It explores an alternate universe where Caius has a more complex relationship with Bella. Another is 'Caius and Bella: A Forbidden Bond' which delves into the challenges they face due to their different natures.
The themes of transformation and self - discovery are quite common. Bella's transformation into a vampire (if it occurs in the fanfic) and how that affects her relationship with Caius is often explored. Caius, too, may go through a form of self - discovery as he gets involved with Bella. Another theme is the struggle against fate. Their relationship is often seen as something that is not meant to be according to the rules of their world, but they fight against that in the fanfiction.