I tend to think of 'bugging out' as an extended, unsupported through-hike. Your concerns will be largely the same so that's a decent place to start; those are (potable) water, food, comfort (shelter). If there really is a significant chance of using the bag keeping some food/water on hand (in or near the bag) is a good idea. Shelter is based entirely on your location. As far as weight, the balance between usefulness/comfort/weight is one you can only figure out on your own.
As important as preparation is, practice is vital as well. Whatever you decide to shove in the bag, take it out and use it some weekend. Then you'll really know what you need.
//edit: One thing I want to add on the procurement of potable water is that
I don't trust Iodine for purification. Boiling is better for eliminating organisms.