The 6' 8" rectangular door is such a standard pattern, and is so taken for granted, that it is hard to imagine how strongly it dominates the experience of transition. There have been times, however, when people were more sensitive to the moment of passage, and made the shape of their doors convey the feeling of transition.
An extreme case is the Japanese tea house, where a person entering must literally kneel down and crawl in through a low hole in the wall. Once inside, shoes off, the guest is entirely a guest, in the world of his host.
Among architects, Frank Lloyd Wright used the pattern many times. There is a beautifully low trellised walk behind Taliesin West, marking the transition out of the main house, along the path to the studios.
If you are going to try this pattern, test it first by pinning cardboard up to effectively lower the frame. Make the doorway low enough so that it appears "lower than usual" - then people will immediately adapt to it, and tall people will not hit their heads.
Test the height before you build it, in place - NATURAL DOORS AND WINDOWS (221). Build the door frame as part of the structure - FRAMES AS THICKENED EDGES (225), and make it beautiful with ORNAMENT (249) around the frame. If there is a door, glaze it, at least partially - SOLID DOORS WITH GLASS (237).