Kim Pine herself is often very well - written. Writers can really explore her complex personality, her love for music, and her sometimes prickly relationship with Scott.
A possible storyline is that Kim Pine discovers a hidden talent of Scott Pilgrim that no one else knew about, like he can paint amazing murals. This could lead to them collaborating on an art project in the neighborhood. Well, in some fanfictions, there could also be a storyline where they go on a road trip together. Along the way, they meet all kinds of strange characters and have to overcome various obstacles, which in turn strengthens their friendship or even leads to something more.
Perhaps there could be a plot where the two worlds start to merge. Scott Pilgrim and his friends find themselves in Konoha, and Naruto has to help them adjust. Scott might get into trouble by accidentally challenging some of the ninja trainees to a fight, thinking it's like his normal battles. Naruto then has to step in and smooth things over. In the process, they could learn from each other's fighting styles. Scott could show Naruto some of his more unorthodox fighting moves, and Naruto could teach Scott about the importance of focus and inner strength in combat.
A fanfiction could focus on a time - travel plot. Knives goes back in time to a point before she met Scott Pilgrim. She then has the opportunity to change the way things happened in their relationship from the very beginning. This would allow for a completely different take on their story, and could introduce all sorts of new interactions and plot twists.
Friendship is a common theme. Since they are friends in the original, fanfictions often explore different aspects of their friendship, like how they support each other through tough times.