In a fanfiction, their friendship could be developed by introducing a new character who tries to come between them. This new character could be jealous of their bond and try to manipulate them. But Mal and Evie, through communication and loyalty, would see through the lies and their friendship would become even stronger. For instance, they could have heart - to - heart talks where they express their true feelings about each other and how important their friendship is to them.
One way is by having them go through a really tough trial together, like being wrongly accused of a crime in Auradon. They would have to prove their innocence and during this process, they would rely on their trust in each other. Their friendship would deepen as they see how far they are willing to go for one another.
Their friendship could be further developed if they decide to start a project together in Auradon, like opening a school for kids from the Isle of the Lost. They would face many challenges, from getting the resources to dealing with the prejudice of some Auradonians. But through it all, they would support each other, share their ideas, and their friendship would grow as they work towards a common goal.