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.
It depends on your perspective. Some might consider it new as it brings in new elements and developments. But in a broader sense, it still builds on the existing Scott Pilgrim universe.
Definitely. 'Scott Pilgrim Takes Off' draws inspiration directly from the comic. It retains many of the key elements and storylines from the source material.
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 is likely to be one of the main characters. He has been central to the previous stories. Ramona Flowers is also probably a main character as she has a major role in Scott's life.