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.
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.
The parents could also be important characters. Spencer's parents might have concerns about her spending time with a detective, while Marco's family background could be explored. Maybe his parents were also involved in some mystery - solving in their youth, which influenced him.
One possible plot could be that Spencer and Marco team up to solve a new mystery in Rosewood. Maybe there's a series of strange events that the police are overlooking, and Spencer's sharp mind combined with Marco's detective skills make them an unbeatable duo.
Well, it's hard to say who the 'best' is as it depends on personal taste. But some popular authors might include those who have a deep understanding of the characters. For example, some authors who have been writing for a long time and have a large following. They are often good at capturing Spencer's intelligence and Toby's more reserved nature.
One interesting storyline could be their future together. Maybe they move away from Rosewood and start a new life, away from all the drama. They could be building a house and planning their wedding at the same time.