One possible storyline could be Spencer and Andrew teaming up to solve a mystery in Rosewood that the other Liars are oblivious to. They could use their combined intelligence to piece together clues.
Character development is crucial. You need to understand Spencer's intelligence and her insecurities, and Andrew's own personality traits. For example, if Andrew is portrayed as a bit of an outsider, it can add depth to their relationship as Spencer, who is part of the main group, bridges that gap.
Another reason is that they both have had their own share of troubles in the show. By pairing them in fanfiction, it gives the writers a chance to show how they can heal each other. Spencer, with her family problems, and Caleb, with his past in foster care, can find solace and love in each other. This aspect of two damaged souls coming together is very appealing to many fans who write and read fanfiction about them.
One common theme is forbidden love. Since there are so many secrets and complications in the show, this often translates into their relationship in fanfiction. Another theme is mystery, as both characters have their own secrets, and fanfic writers like to explore how these secrets affect their relationship.
In fanfiction, Wren and Spencer often have a complex relationship. Sometimes, they are depicted as having a secret, passionate affair. Wren's mysterious nature adds an element of intrigue to their connection, while Spencer's intelligence makes their interactions intellectually stimulating.
I'm not sure who the 'best' is as it's subjective, but some popular authors might be those who have a large following on fanfiction platforms. Look for ones with high ratings and lots of reviews. For example, there could be an author named 'RoseWriter' who is known for her detailed characterizations of Emily and Spencer.
One key element is the character interaction. How Spencer, with her intelligence and complex personality, interacts with Dean, who likely has his own set of skills and traits. Another is the setting integration. Incorporating the dark and mysterious world of Pretty Little Liars with Dean's possible role in it. Maybe he could be helping to uncover the 'A' mystery or dealing with the other secrets in Rosewood.
There are many great writers out there. Some popular ones are often those who are really good at capturing the personalities of Spencer and Toby as they were in the show. For example, some writers on fanfiction.net have a large following for their PLL fanfics. But it's really subjective, as different people like different writing styles.
Another aspect in fanfiction could be a romantic relationship between them. Their personalities, which are both strong and independent, can create a very passionate and intense dynamic. Writers might explore how they deal with their feelings in a world full of secrets like in Pretty Little Liars.
One common theme is mystery. Since the show itself is full of mystery, fanfictions often continue this trend. Spencer and Alex might be involved in solving a new mystery together, perhaps related to their families or the town's secrets.