Holography, the real kind, not the Star Wars kind, is probably the way forward. The display would be about as flat as current ones, but the image would appear three dimensional from any angle and wouldn’t require glasses. You could even lean side to side to change your perspective. I remember reading ages ago that a lab was working on it, but I think they had a frame rate of one every few minutes and it was monochrome and I haven’t heard anything since.