Caroline would also be devastated. As Stefan's friend, she would have to deal with the loss. It could make her more determined to protect the town from any threats, as a way to honor Stefan. Bonnie might try to use her magic to bring him back, which could put her in a dangerous situation, especially if it involves dark magic. The other vampires in Mystic Falls would also feel the impact. Some might see it as an opportunity to gain more power, while others might fear for their own safety as the power balance in the town shifts.
For Stefan, if Katherine dies, it could be a huge emotional release. He has been so tangled up with her for so long. Her death might finally let him move on completely and focus more on his relationship with Elena or just find peace on his own.
For Bonnie, Elena's death could be a huge blow. Since they are best friends, Bonnie might be consumed by grief and use her magic in dangerous ways, either trying to bring Elena back or lashing out at those she blames. Caroline would also be devastated. She would probably try to keep the group together in the face of this tragedy, but also struggle with her own sadness. And for the other characters in Mystic Falls, Elena's death could change the power dynamics. Some might see it as an opportunity to gain more control, while others would be afraid of the chaos that could follow.
Caroline, for example, might be more accepting as she is also a vampire. She could help Elena adjust to her new life. Damon, on the other hand, might be jealous or feel threatened that Stefan made Elena a vampire. It could change the power balance between the three of them.
For Stefan, Elena's death could be a huge blow. He might go into a deep depression or spiral out of control as he had always been so in love with her. Damon, too, would be severely affected. He might become even more vengeful and angry, blaming himself or others for her death.
In some fanfictions, Stefan's character might become more assertive in his relationship with Katherine. Instead of being the one always trying to resist her charm, he starts to take charge and make decisions that affect both of them.
Jeremy's death in fanfiction can have different impacts. His friends in the Salvatore brothers might become more vengeful. They could go on a rampage to find out who was responsible. Elena might struggle with her vampirism even more, as she has now lost her last remaining family member. Some fanfictions might show that Caroline steps up to be more of a support for Elena, filling the void left by Jeremy's death.
It can make their relationship extremely strained. Stefan may start to distrust Damon completely.
Damon's compulsion of Elena can completely change the trajectory of their relationship. It might make their relationship more passionate in some cases. For example, if he compels her to be more honest about her desires, it could lead to a more open and intense connection. However, it can also be a source of mistrust. If Elena discovers that Damon has compelled her without her knowledge, it could seriously damage their relationship and take a long time to repair.
Caroline's pregnancy could also bring her closer to some characters she previously didn't have a strong bond with. For instance, an older vampire or witch who has knowledge about such pregnancies in the past could offer their guidance and support. This could lead to new friendships or mentor - mentee relationships as they all work together to ensure the well - being of Caroline and her baby.
One great fanfiction is 'A New Beginning for Stefan and Elena'. It explores a different path for their relationship after the events in the show, where they face new challenges together in a world filled with both old and new vampires. It's a really interesting take on their relationship and shows a more mature side of their love.