My Wife and I joke that we should test bags for the different manufacturer's. In my years of photography, I've probably gone through about 20 different bags and am always looking for that newest bag that'll fit my every need.
My current one that I've beaten the hell out of for the last two months is the Tamarac Velocity Toploader. I like the way I can throw it over my shoulder and get into it in a few seconds if need be. It'll hold a few extra lenses and maybe a full size 1D battery or two as long as I've got the camera in my hand, as I almost always do. It comes with a sewn in waterproof cover, that has come in handy plenty of times as it's strapped on the top of my kayak when ever we're out paddling.
It has plenty of D-Rings to attach various other things we bring along whil out hiking, and has so far lasted the longest of most of my bags. For the fast approach style of bags, this has to be my favorite. The only thing I wish it had, was a few side pockets, but I think Tamarac made it specifically so you could purchase any of the many accessories that go with it.
I also have the Tamarac large expedition style backpack, but found it was way too heavy unless I'm bringing everything and the kitchen sink with me. I once was flying to Hawaii for a month long photo shoot, and brought it as my carry on. They were very strict as to our weight of our checked bags, but while they werent looking, I laid my backpack on the scale, and was shocked when I saw it weighed 55lbs!! No wonder my back hurt after 3 airports and 18 hours of lugging it around.
Good luck in your design, I'd love to check it out when you have it completed.