Help me come up with a plot for a novel. I have a rough idea of what I would like it to be about, I'd like you to help me come up with an exciting plot. The book is set in two halves, with each half from the perspective of two protagonists, Sylvie and Dan. Sylvie is a young woman, and Dan is a middle-aged man. It is set in a post-apocalyptic world where a mysterious disease has wiped out most of the plants in the world. The book is entitled "Seeds of Hope" 
Washington state, 2029. It has been three years since a mysterious disease, nicknamed "The Reckoning", wiped out 95% of the vegetation on Earth. Widespread starvation and death followed. Towns and cities became desolate concrete jungles. Internet and cellular communication ceased to exist, with radio being the only remaining way for people to contact one another. The world's elite abandoned their people to live a life of luxury and safety in Caelum, a space station orbiting the Earth. Below, the surviving population struggles to sustain themselves. 

Once vibrant cities are now home to three groups of people: Honest survivors, mostly living in groups. Many of them form communes, where they grow and harvest the few remaining species of plants that didn't succumb to The Reckoning, mainly a handful of fruits and vegetables. Then there are gangs of outlaws. Never staying in one place too long, they band together, travelling between towns and cities. Brutal and heavily armed, they will loot, steal and kill whoever they need to ensure their survival. Finally, there are merchants, solitary and nomadic individuals who travel across the country selling anything of value, including rare and extremely hard-to-find packets of seeds. 

Sylvie is a 28-year-old woman living a solitary existence in Seattle. She lost her mother shortly after The Reckoning started. Wary and untrusting, she shunned the idea of living in a commune in favor of going it alone. However, Sylvie has a secret - she is the daughter of a world-renown botanist. Her father was working at the Svalbard Global Seed Vault when The Reckoning hit. The world lost touch with those at the vault shortly after. She knows if her father still lives, he can help repopulate the world's plant life, but the only way to find out is to get to the vault, a seemingly impossible task.

Dan is a 41-year-old former soldier. After losing his wife and sons, he became the head of a commune just outside of Seattle, known as Wisteria. All that matters to him is the survival of those who entrusted him to lead them - until a chance meeting with Sylvie. After he discovers who her father is and her plans to travel to the vault, the two of them set out on a long and dangerous journey to help restore order and life to the world.