If you follow Silicon Valley at VC level, all the rage right now is Augmented Reality (AR) and Virtual Reality (VR)... I know through the grape vine that Musk was debating current AR/VR tech in autopiliot team meetings way back more than an year ago. I haven't heard anything after that, nor do I know any detail. All I know is that Musk was interested in that tech. This was later proven out with public reports that Tesla hired hololens designer. I feel reluctant to hype this up. But I believe the HUD will be something truly ground breaking. Probably far ahead of anything out there, even outside of cars. I think it will propel some crazy amount of sales. But, it's all just pure speculation on my part at this point.
I, on the other hand, don't believe HUD will ever show up in the Model S. I think it would take a fairly major redesign of the whole upper dash area without it looking like an aftermarket add on and this would be too big a distraction from Model 3 ramp up. I believe HUD will show up first on Model 3.
But, I could be very wrong...
Mike