I've been waiting for someone to reflect a HUD off the inside of the dashboard. Having enough brightness for daylight visibility under moderately adverse conditions,, and having it smart enough to modulate that to maintain contract without becoming blinding, is the hardest part.
Yeah, a wearable HUD might be a reasonable alternative, but coming up with one that doesn't block peripheral vision and that's really comfortable enough to drive with seems like it could be a challenge.
My solution was to mount the GPS in front of the tachometer, so it becomes part of the instrument cluster. (Who needs a tach in an automatic? For that matter, who needs a tach in a manual after you've learned the sound of the shift points?)