Their friendship can be developed further by having them share more personal secrets. For example, Hanna could tell Alison about her insecurities regarding her family's financial situation even after things got better. Alison, in turn, could open up about her fears of losing her friends again. They could also have more adventures together, like exploring an old abandoned house in Rosewood and relying on each other during the spooky moments.
To develop their friendship further in fanfiction, they could support each other in their romantic relationships. Maybe Hanna is having trouble with Caleb and Alison gives her really good advice. And when Alison starts to like someone new, Hanna could be there to boost her confidence. They could also volunteer together at a local charity, which would strengthen their bond as they see each other in a different, more selfless light. Plus, they could start a tradition of having a girls' night every week where they just talk about everything.