Since it was unfinished, the full plot isn't entirely clear. However, from the existing parts, we can get some glimpses. It might have been about the relationships and social situations of the characters. Austen was known for exploring love, marriage, and class differences in her works, so it's likely this unfinished novel was headed in a similar direction.
In Austen's unfinished 'Sanditon', Mr. Parker was one of the main characters. He was very much into promoting Sanditon as a seaside resort. Charlotte Heywood was also an important character. She was a young woman who was brought into the world of Sanditon and was likely to be at the center of the social and romantic happenings that Austen might have developed further.
One possible reason could be her untimely death. If she fell ill or passed away unexpectedly, she wouldn't have had the chance to complete it. Another reason might be that she got stuck in the plot development. Sometimes, even great authors face difficulties in figuring out where the story should go next. It could also be that she got distracted by other aspects of her life, like family matters or social engagements.
Jane Austen's last novel was 'Persuasion'. It's a story that delves into themes of love, second chances, and the power of persuasion. The main character, Anne Elliot, is a complex and interesting figure. Austen's writing in this novel, as in her others, is sharp and perceptive, with detailed descriptions of the society of the time.