Please forgive my impudence, but I have an idea I wanted to float to Sir Terry and was unsure of how to get it to him. His official website presented me with an opportunity that I hope will facilitate my intention. I hope that he comes to see my offering just as a suggestion and accepts it or takes it with a grain of salt or otherwise disregards it, as is at his discretion.
Preamble out of the way, I've been reading the Long Mars and, for some reason, an ending to the Disk World series has presented itself to me. While I hope that it may assist Sir Terry, as mentioned above it is only a suggestion and is not intended to offend in indicating that I may know any better than the author himself as to the direction if of which his tale will be woven (bit nervous doing this as it feels like I'm offering constructive criticism to God so just a tincy bit more preamble; sorry).
How the Disk World Ends
Have Rincewind and the Unseen Professors confront Vetinari about the imminent demise of the Disk World. Have it explained that there is no saving the disk world from whatever the calamity may be (perhaps the Sun is dying? Nice reference to Pratchett's own bowing out of life?).
Have the characters come to understand that their only possibility of continuing to exist is to slip into a new reality; a fantasy reality. Slowly introduce to the readers that the fantasy reality is the memories and imagination of themselves, the readers, and that the DiskWorldians have come to understand that the only way to survive is inside of the memories and imaginations of the readers themselves. Show the coming to understanding by the characters through an epic journey by Rincewind and Two Flowers, reunited once again, with their encountering all of the major characters of all the DW books; useful so that every fan has the chance to say goodbye to their favourite character regardless of who they are or from which book they may have been drawn.
Death could release all dead characters so that the whole story of DW could be told; perhaps the characters could come to understand that the whole tale of the DW would need to be transferred, the good and the bad, for the characters themselves to be transferred to the fantasy reality in the form that they themselves are in (we are all remaking ourselves every moment through our every interaction with everyone else so our character is made of everyone who has ever lived while we have, as well as everyone before as they have helped create ourselves and the others around us, kinda thing).
Perhaps overlay the main tale with a chapter opener for each chapter (or maybe every second one?) showing leaves blowing in the breeze or sunlight drifting across a space of white to symbolise the turning of the pages of the book as it is being read. Leave the big reveal of this aspect until the end so as to give the reader an 'Aha!' moment when they realise they, themselves, are in the story as an active participant while also being the passive observer.
It may be possible to give the story immortality through recreating its ending to appear like a never-ending story that lives within each reader. Perhaps have the reader come to understand that each character in the DW series is just another aspect of the readers own personality; that the DW characters have bled through into our world and speak more loudly in others....
I could keep going down this path but I think this already is way more than is necessary to start discussion or to indicate to Sir Terry the almost worshipful adoration I have for his story which has walked beside me my whole life.
I cannot praise his works more highly than to say I will share them with my children and grandchildren so that his stories can live on in my family forever.