I'd like a sequel to Blubber, by Judy Blume: 20–30 years later, during the Halloween period, a glamorous, grown-up Linda tracks down Wendy, Caroline & Jill, and slaughters them! (It would be particularly apt, as the original story commences @ Halloween.) I did once suggest that on a website, but Judy herself said the sequel I had in mind would probably be too gruesome for most readers.
Are there any books which make you feel "I'd like to know what happens next in the characters' lives"?
Joy in the Morning by Betty Smith who wrote A Tree Grows in Brooklyn. That too actually... The manuscript of Joy in the Morning was hugely cut for publication. I dream of a special edition with the missing quarter, but I doubt there would be the demand.