Well, a frequent theme is the power dynamic. Batman is often seen as the more dominant figure in the DC universe, but in the context of a romance with Black Canary, they have to find a balance. Also, the theme of 'hidden emotions' is common. Given Batman's stoic nature, it's interesting to see how he deals with his feelings for Black Canary in fanfiction. And of course, there's the theme of 'partners in crime - fighting and love', where their relationship is intertwined with their work to keep Gotham safe.
In Batman and Black Canary romance fanfiction, a common theme is the struggle between duty and love. Both are committed to protecting Gotham, so they have to figure out how to make their relationship work while still fulfilling their responsibilities. Another theme is the discovery of a new side of themselves through the relationship. For Batman, Black Canary might make him more open and less guarded. For Black Canary, Batman could introduce her to a more strategic and calculated way of dealing with threats. There's also the theme of 'forbidden love' in some cases, as their different lives and the secrecy in Batman's world might pose challenges to their relationship.