When I first started this project, I had the idea that the most robust part of your character should be his capacity to absorb punishment. As I move through this design, I'm all the more convinced of it, because taking damage needs to be somehow interesting too, as that appears to be a big thrust of the game.
And then there's the request that the rules be involved, maybe even a little complicated. Yeah, for real, that was specifically requested.
I'm no fan of making something needlessly complicated, so I'm instead taking that request as permission to go a little more in depth than I might otherwise and not bother sanding it smooth afterward. What's that leave me with?
The damage matrix.
What I've done is create separate health meters for differnet kinds of abuse a survivor might suffer. (Side note: Survivor. I like that as a term for character. I think I'll use that.) So if you're sunburnt and starving, you can't take a beating as well as you normally could. You could represent this from a single damage meter, but that doesn't provide enough detail for a game like this. So instead, you have various metrics of heath: nutrition, health (disease), toughness (physical injury), weathering (exposure), and packing (how much gear you can carry).
As you encounter different hardships, different health meters get assaulted. Tracking damage is pretty easy, and largely similar to how it works in other games. You have a number of levels, and each level can take a number of points equal to your score. So let's say you have 4 health levels in all scores (because it's looking like that's what I'm going to use). If your Toughness is a 5, you can take 5 points in each of those 4 levels. So far, pretty standard stuff.
Here's where it becomes a matrix. As you drop to a new level, you take wound penalties. You wound penalties don't affect your skills or your actions, but your other health meters. You don't take damage in those other meters, but those scores drop. So say you've also got a 5 in Health, and you take a few gunshots that damage your Toughness to the point that you take wound penalties. This drops your Health to 4. Now, you can only take 4 points of damage per wound level. You don't actually take any Health damage, but you're not as resilient to disease until those gunshots heal.
Now let's say that you already had 5 points of Health damage when this happens. Normally that keeps you in the first level, but when your Health drops to 4, those 5 points can't be contained in a single level. Now 4 points fill the top level, and the last point goes into the next level down, which also inflicts penalties. Damage suddenly cascades.
It's a little tricky to track and to explain, but in the sample I showed someone, he got it as soon as he began to use it. I'm thinking this might make a good app for the iPhone to help in tracking, because I've yet to come up with a good way to visually present the damage matrix that makes it both easy to use and understand.
In a nutshell though, that's the core of the character. Survival and pain and suffering, oh my!