A common theme could be family ties, considering their relationship as mother and son in the original Batman lore. It adds an interesting layer to the fanfic.
Another is their journey to find a common ground in their different views of justice. Talia, with her upbringing in the League of Assassins, and Damian, who has been influenced by Batman, try to reconcile their ideas. This often involves complex character development and interesting interactions that have nothing to do with sexual relationships.
The emancipation of Harley Quinn has been depicted as a comic. It explores her character development and story in that medium. There are various storylines and art styles that contribute to the overall narrative.
Harley Quinn's sexuality in the comics isn't strictly defined as gay. It varies across different story arcs, with her showing interest in characters of various genders.