I think if Salinger had intended a 27th chapter he would have included it in the original or subsequent reprintings. The ending is unfinished because that leaves the reader with a certain sense of disequilibrium. We are not meant to experience a happy ending because that would then trivialize the suffering that Holden has gone through. We are left as uncertain about the book as Holden has become about life.

If there were a chapter 27, I believe that it would illustrate that Holden has come through his identity crisis and has finally recovered from his overwhelming grief over the loss of his little brother Allie.

Holden is now attending Pencey Prep again, after his father made a huge donation to the school.  Now Holden can complain about the fact that his father's donation has now resulted in the Caulfield Library. 

Holden has successfully completed his first semester, and the reader is surprised to discover that he is enjoying being a 16 year old boy.  He has made friends with other students at Pencey, Stradlater and Ackley both graduated.

Holden and Jane Gallagher have gone out on a few dates.  He is currently imagining his life after high school.  He wants to be a school counselor so that he can help other kids who are struggling emotionally with depression or grief. 

Holden has survived his depression, but his experiences through the three years that brought him into a deeper and deeper darkness, have given him a great deal of compassion for others who suffer similar fates.