BloodLust
Enlightened
Okay, technically this isn't part of a BOB. Simply supplements it. A slightly large belt pouch or average fanny pack with the basics needed in it. Not my idea. Comes from a student of a wilderness survival expert.
Concept being that should you have to ditch your BOB for whatever reason, you still have the basics that your EDC doesn't cover. For example, a few water purification tabs, a small ferro rod and striker, a decent multi-tool with a saw feature (if you don't EDC one, or perhaps carry a tiny multi-tool as your EDC). Along those lines. Items to only be used if you get separated from your BOB. Perhaps because someone wants it and starts chasing you with ill intent. Might have to ditch it due to its weight slowing down your escape. Or perhaps you decide getting rid of it will satisfy your pursuer. You might be out matched or outgunned. At least with the belt kit that never leaves your body, you still have a few supplies to keep going.
I do this concept with my EDC and when I go camping.
My EDC is a sling bag with the basics that integrate into my BOB when I need it to.
When camping, I have a separate lumbar pack from my hiking bag. For some sites, it's impractical to lug uour whole pack ip to the summit, so the main bag stays in camp and the lumbar pack comes with me.