I would like to see King try his hand at creating a horror-movie franchise for today's generation. I would love to see him try to compete with all the "Saw", "Last Exorcism," "Blair Witch," "Paranormal Activity"-Lionsgate-Entertainment-type stuff out there.
Besides that, would love to see him create a TV series for today's generation, something along the lines of a "Hannibal" or a "Breaking Bad."
In terms of letters, I would love to see him go back to his "Night Shift" roots and write a collection of short tales that contain the hungry voice of a writer trying to establish himself in the field, one juggling to pay the bills. Some of those "Night Shift" (you can probably throw in "Skeleton Crew" as well) stories are extremely entertaining.
Last, I would love to see King write a special anthology that contains short-story sequels/prequels/whatever to some of his novels. What happened after "The Tommyknockers?" Did "It" ever return? What about a story centering on an event in the life of Annie Wilkes? What about a background tale on Raymond Andrew Joubert? What did Louis Creed do after his wife returned? How about a brief continuation of "The Mist," or a prologue to it? How did the grand opening of "Answered Prayers" fare? Just short stories about all this kind of stuff. Perhaps Joe and Owen might want to pitch in on something like this.