Survival is also a big theme. Just as in the Titanic story where characters are fighting for survival during the sinking, in these fanfictions, Bella might be using her skills or knowledge from her own world to help with survival on the Titanic. Or she could be learning about survival in a more historical context which then changes her when she goes back to her own world.
One common theme is love across different worlds. Since Bella comes from a world with vampires and werewolves, and Titanic is a historical love story, the fanfictions often explore how love can transcend these different settings.
One common theme is 'alternate universes'. Writers often explore what would happen if Bella made different choices or if the Twilight world had different rules. Another theme is 'power discovery'. Many fanfictions focus on Bella finding new powers or abilities she didn't have in the original story. 'Love triangles' are also common, where Bella may be caught between different characters in new and interesting ways.
One common theme is self - discovery. Since Bella is in a new form as an elf, she has to figure out who she is now. Another is the exploration of new powers. As an elf, she may have abilities she didn't have before.
One common theme is the clash of two different fictional universes. Bella's world in Twilight is quite different from the magical world of Potter. So, there's often the exploration of how she adapts to the new magical rules and cultures.
One common theme is the crossover of powers. Since Bella has her own unique abilities in Twilight and Supernatural is full of various powers, fanfictions often explore how these interact. Another theme is the clash of different mythologies. The Twilight vampires and werewolves meet the Supernatural demons, angels, etc.
One common theme is the idea of magic. Santa represents magic, and when combined with Bella Swan's world, it often creates a sense of wonder. For example, Santa might use his magic to solve a problem in Bella's life.
One common theme is the clash of different vampire cultures. Bella is from the Twilight vampire world which has its own set of rules, and the Underworld has a very different take on vampires. So, stories often explore how these two different vampire cultures interact when Bella is somehow thrown into the Underworld. Another theme could be power struggles. In the Underworld, there are constant battles for power among different factions, and Bella's presence can either disrupt or become a part of these power dynamics.
One common theme is friendship. The OC often forms a unique bond with Bella, either helping her through tough times or just being a great friend. Another theme is the exploration of the supernatural world. Since the Twilight series has a lot of supernatural elements, the OC usually gets involved in that aspect too, like dealing with vampires and werewolves.
Often, a common theme is the alteration of the Twilight universe. Since Bella is now a god, she can change the rules, the setting, and the fates of the characters. There is also the theme of self - discovery. Bella has to figure out who she is in this new role, which is different from her human self. Additionally, themes of morality may come up as she decides how to use her god - like powers.
One common theme could be identity crisis. Bella is used to being a certain person in the Twilight world, but as a Lestrange, she has to reevaluate who she is. Another might be power struggles. The Lestranges are often associated with dark magic and power, so Bella could be caught in the middle of that. Also, family secrets are likely to be a theme as the Lestrange family has many secrets.
One common theme is the concern of others. For example, Edward and the Cullen family are usually very worried about Bella when she's sick. Another theme is Bella's own struggle, both physically and mentally. She might be worried about how her sickness affects her relationships or her future in the Twilight world.