"Wrist-watches" with bullet and health readouts. Just look down at any point in the game and your virtual arm will come up, showing everything that you need to know on the back of it.

Well, hopefully not for health or ammo. The whole point of a heads up display is so that you can literally keep your head up. A "look down for info" mechanic might be neat for stats you'd normally put on a pause screen or things like your inventory, though.

For health it might not be best, but I think for ammo it would work well. I mean, in the real world you don't even get a HUD for ammo, unless you're in an airplane or tank I suppose. A HUD for health is necessary just because the game isn't real life, but the ammo HUD is just a convenience; you can be more creative with it.

Maybe gun models that give a visual indication of remaining ammo will become more popular.

Perhaps adding blur/color saturation filters to your entire field of view might take off as a primary indication of remaining health.

Amusing to consider due to the viewport, but virtual Glass-type goggles with HUD, rendered within the OR.

Otherwise, yes, watch, checking gun magazine or condition of a blade, or a PDA/map - gaming is going to take a monstrous step up.

Not to mention immersive story-telling as a new way of presenting films.