It enhances the series by adding more content. Simple as that. More stories mean more gameplay time and more things to discover about the Arkham world.
One possible aspect of a Batman and Harley fanfic in the Arkham universe could be their complex relationship. Batman is always trying to uphold justice, while Harley is often caught between her love for the Joker and her own sense of right and wrong. In a fanfic, the author might explore how Batman could potentially save Harley from her self - destructive path. For example, Batman could use his detective skills to show Harley the truth about the Joker's abuse towards her.
One of the best moments could be when Batman manages to outwit Harley in a battle of wits. He uses his knowledge of her psychology to predict her moves. For instance, if Harley tries to set a trap using her knowledge of Arkham's layout, Batman quickly figures it out and turns the tables on her.
One possible fanfic could be centered around Batman's encounters with Poison Ivy within the Arkham universe. Maybe it could explore their complex relationship, with Batman constantly trying to stop Ivy's environmental extremism while also seeing the good in her intentions at times. For example, Ivy might be trying to protect a particular area in Arkham City that she believes is crucial for nature, and Batman has to find a way to balance the law and her motives.
It depends. Different Batman novels might have different statuses regarding canonicity. Some might be directly tied to the main continuity, while others could be standalone or in alternate universes.
Yes, Batman Detective Comics is generally considered canon within the Batman comic universe. It contributes to the overall continuity and lore of the character.
There are indeed non-canon Batman comics. These can range from one-shot specials to alternate reality tales. They give creators the freedom to play with the Batman mythos in ways that don't fit within the regular canon framework.
It contributes by expanding the lore of the Arkham series. The Gotham City stories introduce new characters and scenarios that are related to the overarching theme of Batman's fight against crime in Gotham. This enriches the experience for players who are already fans of the series and also attracts new players who are interested in the Batman universe.