What nobody seems to be chasing (I assume because screens are flashier) is smart headphones. Imagine you could navigate a webpage with hand gestures and have it read to you while you walk or do chores. In some ways, this is way easier than head-mounted graphical displays; portable audio is already a solved problem. The problem that still needs to be solved is good quality TTS on a portable device, but honestly that seems way more tractable than portable HUDs.