I agree that Ponyboy is going to be an author, but I think it's going to take him several years to be successful in writing. He'll edit his story about the guys with his English teacher and then work his way through the first couple of years of college, struggling to publish, before he manages to earn any income from his writing.
Darry will start taking classes at a community college in the year after the story. The events of The Outsiders have changed him almost as much as they have changed Ponyboy, and Darry is no longer willing to accept his rough place in society. However, Darry's school career will be difficult, and he'll be unable to give it his full attention until Ponyboy graduates from high school.
Sodapop will experience a period of unhappiness as his brothers pursue their goals. He'll feel a little guilty about this because he knows he wants them to succeed, but he'll be sad that he is losing the life they had together in the novel. Eventually Sodapop will get married and have a couple of kids, and that will be the way he reaches contentment. He'll be a great husband and dad.
I think that Ponyboy is going to be a famous author of books for teens. His book that he is starting for his theme will be very famous and make him rich. He will spend time helping kids who he think are like he was 10 years ago.
Because he's got money, he's going to help Darry be able to go to college. After college, Darry's going to be a management type because he is good at leading people.