It may also explore the theme of commitment. The relationship between the two main characters would involve a level of commitment, whether it's emotional, physical or in terms of building a life together. This could be shown through their interactions and decisions in the novel.
Love is surely a main theme. It likely explores the different aspects of love between two people who are in a relationship that could lead to marriage.
Love and loss are likely main themes. The title itself 'Goodbye My Lover' implies that there is a parting in a love relationship, which is a common aspect of stories dealing with love and loss.
Love is likely a main theme. It's about a special relationship between the narrator and their dream lover. Another theme could be longing. Since it's a dream lover, there's probably a sense of longing for that person. And perhaps self - discovery. Through the relationship with the dream lover, the main character might discover more about themselves.
Perhaps commitment is also a theme. In a relationship with a lovely wife, commitment would be an important aspect. The characters might show how they are committed to each other through different situations in the novel.
Love and loss could be main themes. Often in novels about a first wife, the relationship that was once there and then lost is a central aspect.
Growth is often a theme in such novels. After the experience with the previous lover, the characters usually grow and change. They learn from the relationship, become more mature, and might approach future relationships differently.
One main theme is probably power dynamics in relationships. Since the wife is a CEO, there's a lot to explore about how power is balanced in their marriage.
Love. It's likely a central theme as it's about a relationship with a tycoon.
Another theme could be about redemption. Maybe the character Landon is trying to redeem his or her lover who has a dark past as a killer. It could be a story of trying to change someone for the better.
One possible theme could be devotion. If a wife is considered as one's life, it implies a high level of commitment and loyalty. Another theme might be the significance of the relationship in shaping one's identity. Since the wife is equated to life, she probably has a major impact on who the person is.
I'm not sure as I haven't read this novel specifically. But generally, it could be about the discovery of the special and hidden qualities of a lover.