It could also be about identity. The traveller might struggle to find their place in both worlds, not fully belonging to either, and the story could revolve around their journey to define themselves within this unique situation.
Perhaps there is an antagonist in one of the worlds who opposes the traveller's presence or actions. This could add conflict and drive the plot forward as the traveller has to overcome the challenges posed by this character.
The main theme could be about the struggle between two different realms or ways of life. It might explore the challenges and growth of characters who are caught between these two worlds.
The main theme could be about the collision of different worlds, perhaps representing different cultures, ideologies or even different dimensions. It might explore how characters deal with the chaos and change that comes with such a collision.
The title 'the worlds between us' gives the impression that it might deal with the divide between individuals or groups. It could focus on how these 'worlds' are created, what keeps them apart, and how the characters in the novel either overcome or are trapped by these in - between 'worlds'. It might also touch on themes of isolation, connection, and the human experience in relation to these different 'worlds'.
I'm not sure specifically as I haven't read the novel, but usually in a story about two worlds, there could be a protagonist who is a bridge between the two. Maybe a character who was born in one world but has to enter the other for some reason.
The male lead from the webtoon world and a female from the real world are the main characters.
Love. The story mainly revolves around the complex and profound love between the main characters.
The main theme might be about love and loss. Since it's called 'Two Broken Hearts', it could be centered around two characters who have experienced heartbreak in love, perhaps dealing with betrayal, unrequited love or the end of a relationship.
The main theme could be love. Since it's about the journey of two hearts, it's likely centered around the relationship between two characters, perhaps their growth in love, the challenges they face together, and how their hearts change over the course of the story.
The main theme could be about the complex relationships between the two Mrs. Grenvilles. It might explore how they interact with each other, their families, and the society around them.