I was just about to post a similar thread lol. I'm thinking about the shapeshifter also by think tank to go with a belt system.
When you've got your gear out and on the belt, with cameras around your neck and little or nothing in the backpack, the shapeshifter compacts itself down to only 3 inches thick on your back. The downside to the bag is it's not an active shooter bag(can't quickly take things out and shoot with lenses already attached), but since the belt is the active shooting component I think this would be a great combo. You just do all the shooting, not hindered by the empty bag, and then stick everything in the bag when you're done.
I've always had this big tamrac bag that I call my turtle shell because even when it's empty it's a good 12 inches or thicker. So this is what I'm thinking my next bag combo will be.