It might include a vampire hunter who is on Jeremy's trail. Also, Jeremy's family members could be important characters as he tries to protect them from the vampire world he has now become a part of.
Well, in 'jeremy becomes a vampire fanfiction', perhaps Jeremy is turned into a vampire by a mysterious vampire who has some special plan for him. As a vampire, he discovers new powers and abilities. He might get involved in a vampire society full of politics and rivalries. He has to learn the rules of this new world and figure out if he wants to be a 'good' vampire or not.
Obviously, Jeremy and Anna are the main characters as it's in their name. Jeremy is a human with connections to the supernatural world, and Anna was a vampire. Their ghosts in the fanfiction play important roles.
Another interesting pairing could be Jeremy and Caroline. Caroline is already a vampire, so she could mentor Jeremy in a way. They could have a fun, flirty relationship where they go on adventures together and learn more about the vampire world as a team. It would be interesting to see how their personalities clash and mesh at the same time.
Jeremy in the 'jeremy monster story' might be a young adventurer. He could be someone who is curious and that's why he gets into a situation with a monster. For example, if the story is set in a magical land, Jeremy might be exploring and accidentally disturbs a monster's lair.
Perhaps a plot where Jeremy uses his new vampire powers to protect Mystic Falls. He becomes a sort of vampire - hero. He could use his enhanced speed and strength to fight off other supernatural threats that the town faces, all while trying to come to terms with the fact that he's no longer human.
Besides Yui, there could be a group of young vampires who take Yui under their wing. They could be fun - loving and introduce Yui to the vampire nightlife. Then there might be an older vampire mentor figure who tries to guide Yui in controlling her powers. And a human love interest from Yui's past who still has feelings for her and tries to bring her back to the human side.
One great 'Vampire Diaries Jeremy Fanfiction' is 'Jeremy's New Path'. It explores Jeremy's journey after he discovers new powers. It's really engaging as it shows his struggle to control these powers and how it affects his relationships with the other characters in the show.
One great Damon and Jeremy fanfiction is 'The Bond Beyond Blood'. It delves into how Jeremy, after his experiences in Mystic Falls, starts to see Damon in a new light. Their relationship evolves from one of distrust to a sort of begrudging respect and then to an unexpected friendship. The story is full of action and the typical Vampire Diaries drama.
One common theme is friendship. Despite their differences, many fanfictions explore how Jeremy and Tyler could become friends or at least allies. Another theme is redemption. Tyler, especially, has a lot of room for redemption in fanfictions.
Elena would definitely be one of the first to try. She loves Jeremy and would do anything to save him. She would probably start by looking for Damon's usual hiding places.