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.
One great 'Scott Pilgrim x Wallace Wells fanfiction' could be about their adventures in a parallel universe. In this story, they might discover new powers and have to fight against an unexpected villain together. It could show their relationship evolving from just friends to something more complex.
Scott Pilgrim was indeed a comic first. It had a dedicated fan base from the comic world before branching out into other entertainment areas. The comic's success paved the way for its expansion into various mediums.