One common theme could be forbidden love. Since there are likely some barriers or complications in their relationship within the original story context, this makes for an interesting and often - explored theme in fanfiction.
You can try fanfiction websites like Archive of Our Own (AO3). There are a large number of fanfictions there, and you can use the search function to look for 'annabeth loves luke' specifically. Just be aware of the site's rules and guidelines.
One good example could be a story where Annabeth's love for Luke is a slow - burn. It might start with her admiring his confidence and leadership skills in a mortal high school setting. Maybe he's captain of a sports team and she's drawn to his determination. Another example could be a fanfic that delves into the internal conflict Annabeth has because she also has a friendship with Percy in this AU, but her heart pulls her towards Luke. A third type could be a story where Luke is in some kind of trouble and Annabeth's love for him drives her to go to great lengths to save him, even sacrificing her own relationships with others in the process.
I really like 'Luke and Annabeth: The Lost Years'. It delves into what could have happened during the time that wasn't fully explored in the original series. It has some great character moments between them and really builds on their complex dynamic.
One way to portray it uniquely is by having Annabeth's love for Luke be based on a shared interest in art in the mortal AU. Maybe Luke is an aspiring painter and Annabeth is drawn to his creativity. Another unique portrayal could be that Luke has a secret musical talent and Annabeth falls for him when she discovers it during a local talent show. Or, in this AU, they could be partners in a community service project and through helping others, Annabeth realizes her love for Luke.
One great 'Percy, Annabeth, and Luke' fanfiction is 'The Bonds of Prophecy'. It delves deep into their complex relationships. Percy's loyalty, Annabeth's intelligence, and Luke's inner turmoil are all explored in a really engaging way. The story also has some great action scenes that keep you on the edge of your seat.
One great 'Percy, Luke and Annabeth Fanfiction' is 'The Lost Hero's Tale'. It really delves into the complex relationships between the three characters. It shows how Percy's loyalty, Luke's conflicted nature, and Annabeth's intelligence interact in different scenarios.
Luke might become more enraged. Since Percy has thwarted his plan to do something to Annabeth, he could seek revenge. He might start plotting more devious schemes against Percy and Annabeth, or even their friends.
One common theme could be the exploration of their relationship in a more intimate way. Since they have a complex relationship in the original series, fanfiction might take it further. Another theme could be the power dynamics between them, how they interact on a more personal level.
The characters themselves are very interesting. Luke has his own tragic backstory and Annabeth is a strong and intelligent character. Fanfiction allows for exploring how they interact in various situations that we didn't see in the books. It could be a simple moment of them just talking about their dreams or something more intense like facing a new threat together. There are so many possibilities.