Wow, where to start!? I would suggest first to get a copy of Cody Lundin's books 98.6 degrees The art of keeping your a$$ alive and When All Hell Breaks Loose. Cody is the barefoot guy on Discovery's Dual Survival and a very knowledgable guy.
A BOB is useless unless you learn/know what to do with it and basic survival skills!

These books cover all the elements of human nature and then delve into gear and what to use/not use.

1st. WATER....containers, purification pills, purification straws, pot to boil water. Basically several different means to carry and purify water. Cody's books explain this as well as what unsafe water will do to your system etc.

2nd. Fire making. Have several different methods! Storm matches, lighters, magnifying glass, 9volt battery and steelwool.

3rd. Shelter. I picked up some tube tents online that are the basic shelter. Also Ive several military ponchos and a cheepy blue tarp with steel eyelets to tie off

4th. Things you need to prevent hypothermia, correct clothing NOT COTTON. Heat packs space blankets etc.

Ive made 3 BOBs for my family and they all have the 4 basics and duplicated items. If one bag is lost with the firstaid kit the other bags have one as well.

Here are the other things I have in my UGH heavy BOB
Batteries...try to keep gear that runs off similar battery types.

A radio that picks up shortwave and AM and is crank powered

A knife, a utility tool, a trowel or small shovel, Gloves, meds, some food (one MRE per bag), flashlight and headlamp, sewingkit medical kit, 550 paracord 100' per bag, a dogleash, zipties, metal snares for small game, a wrist rocket for game/protection, glow sticks, duct tape, compass, bungee loops (have a small ball at one end), an old hunk of bed spread I found that makes an excellent towel or ground cloth, Oil for guns and equipment, my harmonica, personal toiletries, fire starting kit that has all the stuff to get a fire going, pen and small notebook, big sharpee, change of underware and socks plus a polypro undershirt, bandana, Shemag, its that towel thing you see wrapped around terrorists heads, small fishing kit.

A BOB has to be basic and on the lighter side. Ive packed cache bags that hold the overflow that complements the BOB but isnt really needed.

Remember, the more you know; the less you have to carry. Plus, know/practice using what you pack.

Great luck!