Smallville has elements of the Superman mythos, but it's more of a unique take on the character's early years. It's not the sole authentic version of his story. Other comics and adaptations offer different perspectives.
A fanfiction could also explore a scenario where they are pitted against each other. Perhaps due to a misunderstanding or a spell gone wrong. Zatanna's powerful magic against Black Canary's combat skills would make for an intense battle. But in the end, they realize they've been tricked and come together to face the real threat, thus deepening their understanding and respect for each other.
Fanfiction often gives fans the chance to explore different relationships between characters. For Black Canary and Zatanna, it could be about exploring their potential as a duo in the DC universe. They could be in a story where they are the last hope against a powerful villain who is immune to normal attacks but vulnerable to a combination of Zatanna's magic and Black Canary's sonic scream.