From what we have so far, the main lot seems to be a classical triangle love story where Lizbeth, John and Willy as the three protagonists.
This story ends with the brutal death of Lizbeth. Is that love story the main plot or simply a subplot that will introduce a riveting “Who dunnit” kind of narrative or is Lizbeth’s untimely death the tragic conclusion of a dark romantic drama? Determining what the story is about is the way to chose which side is the main plot and which the subplot.
Let’s pick up the crime narrative as the main plot.
A romance between Dirk Gentle, the detective, and Jane, as planned as the beginning by the writer, adds very little to the main story line. On the contrary, it slows down the action. Better scrap that altogether.
The love story between Lizbeth and Willy, however, is an important subplot that provides ample opportunities to show Willy’s faintheartedness and general lack of will and Lizbeth characteristic victim’s behavior.
The market is a main scene of the story. Dirk Gentle will have to find the clues to establish John’s guilt from there, so the friendship between Jane and Jack, the dairy stand seller, needs to be developed. Without establishing that relationship before the occurrence of the crime, Jane will not have the ability to bring Dirk thee clue that will finally exonerate Willy and bring John in front of the judge.
Subplots are crucial. They provide opportunities to further define the protagonists’ personality, which is important. Yet, if they are not tied logically to the story line and do not contribute to further the action, they only get the attention of the reader away, and the book will be tossed away unfinished and will not be recommended to friends.
So actually, ensuring the relevance of subplots is the first marketing step 🙂