There are plenty of monochromatic cases. Right now hw has a lot of orange.
Dynamic resolution / subpixel rendering. Retina looks really good already, not sure if the effect would be relevant or interesting but it might open up something new
You make it sound like there is still an easy to spot difference. When i look at the print quality of pictures on a news paper, its the opposite and at least for me, i don't need more than retina and i was very eager to switch to 4k to have higher dpi.
But 14' with retina im very happy.
I'm actually more surprised by hdr on my lg oled 4k. Its actually quite nice when done well.
Newspapers are famously printed on the lowest quality recycled paper and cheapest print process available, because they're disposable. Compare a retina screen to a coffee table style reference book with high resolution photos – the kinds you can use a magnifying glass on - and you'll still notice differences.
Or just look at what companies do when manufacturing technologies allow them to push for higher densities: iPhones now exceed 450 dpi, and the 8" iPads exceed 300; if the technology allowed it, Apple would most likely introduce higher densities on larger iPads and Macbooks as well.
Dynamic resolution / subpixel rendering. Retina looks really good already, not sure if the effect would be relevant or interesting but it might open up something new