In fanfiction, the Joker and Harley Quinn can have a huge impact on Gotham. Their antics can range from small - scale chaos like random acts of vandalism to large - scale threats like trying to take over key parts of the city. They are often portrayed as a force that the police and Batman constantly struggle to contain.
Overall, Gotham has some similarities to the comics, but it also makes significant changes to characters and plotlines to make it more unique for the TV series. For example, some characters' personalities and backstories are altered.
In most cases, Gotham is canon to comics. It incorporates many elements and storylines from the comic universe, maintaining consistency and respecting the source material. However, there might be some creative interpretations or minor deviations for the sake of the TV show's narrative.
Sure, Joker delves into the origins of the character, showing the events and circumstances that shaped him. It provides a deeper look into his psyche and the factors that led to his transformation.