We learned in Book 3 what different characters Boggarts become, but not all of them. I'm pondering what some of those we didn't see would become. This is only in reference to if they saw a Boggart around the time of "Prisoner of Azkaban", certainly later on the series tons of characters would see Lord Voldemort instead...
Hermione: Well as Ron joked a paper with a Failing grade on it is a possibility... more likely though it would be an authority figure degrading her for personal failure. Perhaps her parents for example. Alot of her behavior would make sense if she was raised to be terrified of disappointing them. Another interpretation could be Dumbledore telling her she's expelled...
Draco: I'm almost positive he'd see an enraged Lucius
Snape: Lord Voldemort (at this point his greatest fear would probably be Voldemort's return an him seeing Snape as a traitor)
Dumbledore: dead students
Sirius: After all that time in Azkaban, he'd likely see a Dementor
Wormtail: as of PoA his greatest fear was probably being found out & held accoutable for his actions, so perhaps an Auror? Or maybe James & Lily confronting him with his betrayal...
Ginny: an anonymous diary...
What are your thoughts?