Hazel dies shortly after the events of the book.Gus's eulogy was a sort of double ending. It was a clever plot device serves as closure for both of their storylines, as it is the thing that punctuated both of their lives. Since we know what the eulogy says and how Hazel responds to it, we know what happens before and when Hazel dies. Since Hazel is satisfied with her life choices, I get the sense that she's confident about facing her own death.
Hazel writes her own Fan Fic in the An Imperial Addiction universe.It's a crossover in which the Dutch Tulip Man is actually a secret agent who's been trying to track down a rogue botanist who's developed a deadly new strain of Dutch elm disease, and shortly after Anna's death, he teams up with Staff Sergeant Max Mayhem to bring him to justice.
Hazel discovers that she's pregnant shortly after the events of the book.When she had sex with Augustus, he knocked her up, and he died before she could find out about it.
Nat Wolff will vlog about playing a blind character, and the set in general...with his brother, in the style of a John Green Vlogbrothers video.
Hazel is an Unreliable Narrator.She sees herself and Augustus as sophisticated, worldly and erudite, while she has huge gaps in her education and you can see through the cracks so to speak that healthy Gus' appearance ranges from attractive an an awkwardly adolescent way to, when brandishing his unlit cigarette, absolutely ridiculous.
Augustus is the anti-Holden Caulfield.From their background (Holden's midcentury/Silent Generation, East Coast, prep school, hanger-on of the fencing team; Gus is millenial, Upper Midwestern, public school, former basketball star) to their affectations (Caulfield hates "phonies", Hollywood and the word grand, Waters is acutely aware that he is playing the role of Augustus Waters and his first word in the book is "Grand") to their aspirations (Holden wants to stay a kid, Augustus wants to be remembered as a Great Man but dies while still a boy). Heck, even their authors (Salinger was famously a recluse, Green a social-media phenomenon).
John's role in The Film of the Book will be...Lidewij. He'll be dressed in drag during the whole thing. More seriously, he'll be the priest at Augustus' funeral (since John Green was once a chaplain), or in some other way linked to Gus' death.
In-universe, The Fault in Our Stars was written by Van Houten.Hazel encourages him to sober up and write another book. Having met someone that reminds him so much of his own daughter, Van Houten takes what he knows about Hazel and Augustus (from what he learned when he met them in Amsterdam and when he went to Augustus's funeral, which would explain how he knows about Isaac), fills in the gaps with his imagination, and writes TFiOS.