It really depends a lot on you, where you are, and what you want the thing to do.
The first consideration is what you want the bag to do for you. Is it to get you home in an emergency at work? Is it to get you from home to another specific location? Is it something to have on you all the time, just in case? What sorts of emergencies do you want to prepare for?
With this goal in mind, consider the things you're likely to need to accomplish. Are you likely to need to deal with rain? Might want a poncho of some sort. Will you need to be able to hunt? Then you're going to need tools to do that.
Basically, while everyone needs to worry about breathing, shelter, water, food, and injury, how you deal with them depends on your abilities and location. How far you intend to travel and how long you're planning to stay out will affect how much weight you want to carry.
So, what sorts of scenarios do you want this thing to be good for?