Where would the characters be now? How would they be reacting to what happened to each of them at the end of the 18th chapter? How would Hemholtz and Bernard be handling their exile? How do Linda and John's stories continue?
It would be my best guess, and only that, that the World State would forget their existence in minutes. After all, they are trained not to feel: "When the individual feels, the community reels." I'm not even sure how much "impact" they all had on the World State during their time there. They were interesting freaks; but how long does the "odd" live even in our society ... and we tend to thrive on it.
I can't imagine another chapter. The end of the book is the logical outcome of all that has happened. The exiles are not interesting (although this might be another book), and who is left to care about Linda and John? We've off to sex, soma, and "everybody's happy now."
I can see a fatal cataclysm destroying everything as the only reasonable conclusion to a book where everything has happened and everything turned out well.
If a nineteenth chapter is added to Brave New World, its content would very much depend on the choice of the author of the book. Let us imagine that this author is Aldous Huxley himself, coming back to life in the year 2009 and observing all the changes in the World since writing of the book. The question is now more specific - what would Aldous Huxley write in the nineteenth chapter of Brave New World, having obtained a good understanding of the actual developments in the last 80 years or so?
I believe, Huxley would take note of all the positive developments that have taken place and will be more hopeful about future of the world and present a more sanguine picture. Just one chapter will be too short to present a detailed account of how the BNW once again reinvents all those lost human values. So the chapter will just talk about how the suicide of John triggers an introspection among some people in BNW, which is just a faint glow on the Eastern horizon before a new dawn.
I'm not asking if you could imagine another chapter, I'm asking, if there was another chapter, perhaps following their reassignment to the Falkland Islands and how they feel about their new "home"? Anything! How has the World State reacted to everything that has happened? John's suicide? Bernard and Hemholtz's exile? Does the World State just go back to normal after all the abnormal events that have happened? After they have seen what a savage is like and that their old director was a father and a husband? How has it changed and how have the characters changed?