Here is something that I believe is pretty cool: concept art of the Florida Magic Kingdom's Space Mountain. This scan comes from the last page of a pictorial souvenir dated 1972.
What makes it unique? Look closely and you can see the large entrance area- similar in size to the entrance to the Land pavillion at Epcot, maybe larger. An even closer inspection reveals rockets circling the outside of the mountain. To my understanding, these rotating rockets were only for show and illusion with no intent to have thrill seekers actually go outside the structure.
This same souvenir guide also has renderings for Western River Expedition, an unbuilt Marc Davis masterpiece in the tradition of Pirates of the Caribbean (now vaguely rumored for Disneyland Paris); Thunder Mesa, the huge complex that would include Expedition, Big Thunder Mountain Railroad, and other smaller attractions; and a newer version of Pirates itself. Of course, these are all stories and scans for another day...
(Art copyright The Walt Disney Company.)