I love feeling immersed in the survival aspect of DayZ the most, and having a new mechanic which increases player survival risk through health and leaving a somewhat trackable trace would increase this aspect a lot.
I see this mechanic being a timed (hourly or after eating/drinking a lot) process which involves seeing a new stomache icon show up that signals that your character must defecate. Your character must pick any outdoor ground on the game and begin to defecate. The defecation will leave a spot on the ground that remains for 15-30 minutes, and can be "seen" by other players by creating a small yellowish/gaseous cloud around the area. It will not drain any food/health/water from the player and actually prevents the following:
If the player continues to ignore the icon, the player will get a new/old sickness that will cause them to throw up repeatedly as a consequence, draining a significant amount of water and/or food until cured or the sickness goes naturally.
I believe that this mechanic greatly reduces the perma-camping aspect of the game, forcing people to move to a "safe" spot to defecate or before entering combat areas to avoid the consequences after 1 hour. It will also immerse the far-away base builders by forcing them to have a biological timer, which will usually make them move away from their indoor base. A final touch would be to allow the defecation to be buried to completely hide any trace, thus immersing players even more who play extremely stealthy.
And one final suggestion would be to make defecation at building outhouses be untraceable.