Popular themes for titles across genres include:
Romance: Love, relationships, heartache, or redemption.
Science Fiction/Fantasy: Exploration, new worlds, technology, magic, or adventure.
Mystery/Thriller: Deception, secrets, investigations, or conspiracies.
Horror: Fear, nightmares, haunted locations, or supernatural occurrences.
Nonfiction: Self-help, biographies, history, or informative topics related to the subject matter.