Suffocation I think works OK as written; not the most realistic, but I've heard
enough stories of people surviving for relatively long periods without air and recovering that I'm OK with some fudge factor in my D&D games.
Here's how I plan to handle thirst:
Part of what a "short rest" entails is replenishing fluids. If you don't have water (or a reasonable substitute, sometimes fruit or other foods will do the trick), the first surge you spend during a short rest will have no effect, and you will be unable to regain that healing surge until you have drunk some water.
With this in place, characters will often be down a couple of surges before the 3-day mark (where the normal DMG rules start kicking in). It means that characters who exert themselves less will have a greater staying power. It also allows healing powers to help stave off the effects of thirst (i.e.,
healing word functions normally during rests as described in the PHB, it's reasonable to assume that it provides magical relief; warlord's give us a bit more of a narrative problem, but I'm okay with flavoring their
inspiring word as tips and tricks for prolonging survival). Most importantly of all (to me) is that it's very simple to use.
I'm a lot less worried about hunger for some reason - nearly any scenario I can come up with where it would matter I'd pull off as a skill challenge (either "find enough food to survive, if it's the PCs that are starving, or "find the NPCs before they starve" if the PCs are rescuing someone else). Instead of 3 failures being loss, we'll have each failure cost a surge (or similar).
At any rate, the reason the rules look the way they do (I think) is because of NPCs - a heroic tier NPC has to lose only one surge before taking HP damage and starting to die. PCs are, of course, more badass than this and can survive longer.
