Another theme is often the character's journey in a different light. A female Frodo might face different challenges or be perceived in different ways during the quest to destroy the ring. This can add new layers of depth to the story and her relationships with the Fellowship members.
The dwarves who had the adventures with him might also enjoy listening. It would be a way to relive their shared experiences and remember the comradeship they had. They might sit around a fire and listen as Bilbo recounts their journey to the Lonely Mountain.
One common theme is mentorship. Bilbo often acts as a mentor to Frodo, passing on his knowledge and experiences. Another theme is friendship. Their bond as hobbits is explored in many fanfictions. And of course, the idea of adventure also runs through a lot of these stories, as both are associated with adventure in different ways.
Another theme is the passing of the torch. Bilbo is introducing Frodo to a world he will soon be a big part of, so it's about the transition of responsibility. In many such fanfics, we see Bilbo slowly handing over his role as the 'local hero' to Frodo. And there's also the theme of family. Bilbo and Frodo have a family - like bond, and this is often explored in the fanfics. For example, Bilbo might tell Frodo stories about their family's past, which gives Frodo a sense of identity and belonging.
One idea could be that Female Bilbo Baggins joins a traveling caravan by accident. Maybe she was exploring the Shire and got separated from her group, and the caravan took her in. She could then use her wits and hobbit charm to make friends within the caravan, and they could have all sorts of adventures together, like facing off against bandits or discovering a hidden magical place.
A major theme in these fanfictions is the relationship changes. His friends, like the dwarves or Gandalf, might react differently to him as a vampire. Also, the theme of temptation is present. The urge to drink blood and how Bilbo resists or succumbs to it is often explored in detail. For example, in some stories, he has to find alternative ways to satisfy his vampiric thirst without harming the innocent. This adds a layer of complexity to his character and the story as a whole.
One possible theme could be a sense of betrayal. For example, if Bilbo feels betrayed by his companions during the adventure, it could lead to his anger. This could be related to issues like sharing of the treasure or decisions made during their journey.