To make later supplements attractive in print form, I'd love to see important, major supplements get the full-sized, hardcover, full color, big-book treatment. Ptolus!!! Well, maybe not quite that big, but big and meaty, no more of those skinny hardcovers from 4E. Imagine the next Manual of the Planes, or Draconomicon, being a true TOME! Again, expensive, sure.
I like that idea. Also, I would quite like to eventually see an "Encyclopedia Version" of the game - the game, complete and entire, presented across several large tomes, with all the races gathered together, all the classes gathered together, all built for easy referencing...
Of course, that's a dream. But it's a pleasant one.