One popular theme is the exploration of the relationship between the main characters in different scenarios. For example, 'what if' situations that didn't happen in the original book.
Love and relationships are common themes. Since Ali Hazelwood's works often center around romantic relationships, fanfics tend to explore different aspects of love between the characters, like how they overcome obstacles.
Fanfiction.net might also have some. However, the quality can vary a lot more compared to AO3. You'll need to be a bit more patient when searching through the results there. But it's still worth a shot if you can't find what you're looking for on the other platforms.
There are fanfics that expand on the characters from Ali Hazelwood's books. For example, some might focus on the backstories of the main characters, like Olive and Adam. Check fanfic communities on platforms like Tumblr as well, where fans often share their works or links to other platforms where they post their Ali Hazelwood fanfic.
You can start by looking on fan - fiction - specific websites like Archive of Our Own (AO3). It has a large collection of fan fictions from various fandoms, including those related to Ali Hazelwood's works.
There are often fan fictions that focus on the 'what if' scenarios. In the context of Ali Hazelwood's works, it could be like 'What if the characters met in a different setting?' Some fan fictions might take the existing storylines and give them a twist, perhaps by changing the order of events or introducing a new sub - plot that still stays true to the essence of Ali Hazelwood's writing style.
You could have a bittersweet ending where the couple parts ways but with a sense of growth and acceptance. Maybe one of them has to move away for a great opportunity.
You could consider having one of the characters face a life-changing situation that makes the relationship impossible to continue. For example, a job offer in a different city or a serious illness. This creates a sense of realism and inevitability.