An ideal camera bag is something all photographers search for; so far, mine is hiding. A recent on-line question brought some ideas forward, though. I've currently got six camera bags, only two of which get extensive use. Both are shoulder bags, one fairly small Lowepro worn as described below, and one medium-sized Tenba for times when more lenses are needed. The Tenba is a well-made bag, but the strap is not changeable and is too short for cross body use.
One that doesn't get much use is a sling bag, with a sling that's too short and another is a backpack, which I dislike intensely. The rest are shoulder bags, with a cheap Lowepro seeing the most use, as I like its features more than the rest, though it has its faults. The Lowepro meets many of the below specs quite nicely, including a shoulder strap long enough to be used as a cross-body strap.
Age As A Factor
As I get older, I find less desire to carry a lot of gear when I have to walk. In the past, when I was shooting in the field, I'd have two bodies and five lenses, plus a fairly large pile of film canisters, filter wrenches, filters, and similar accessories. Today, on a race course or similar venue, I prefer to leave any back-up body and lenses in a bag in my vehicle, and carry two or three lenses and one body. Lighter is better.
Less Bulk Today
Of course, film is no longer a bulky item, as a 16GB SDHC card gives me about 700 frames of shooting on my Pentax K20D's raw setting. That's equivalent to about 20 36 exposure rolls of film, and sufficient for most days. I carry one more 16GB card and three 8GB cards for weekend assignments, and just in case something interesting happens. The five cards, in their cases, take up about as much space as a single film box used to take, while letting me collect almost 2,500 images, about 70 rolls of film worth.
For a personal bag design, I'd completely lose the backpack idea. Shoulder bags allow more utility, or so I think. Add a cross-body strap instead of a standard shoulder strap, and you've solved the single worst shoulder bag problem: strap slippage. The primary differences are strap length and the shape of shoulder pad.
I started hating backpacks more than 50 years ago, at Parris Island. I've since come to believe that carrying expensive photo gear on one's back in certain areas of the world is a great way to enrage your insurance company. It's also hard to get to when you're shooting ever-changing action.
Materials & Color
Neutral colors are best, nothing bright so olive drab, dark gray, dark khaki, dark green are all good. Blaze orange isn't, nor is light blue, pink or similar light colors that show dirt and attract attention from the wrong types. Some photographers go so far as to use diaper bags for their gear, but I'm not interested in that kind of misdirection--I do not let my bag out of my sight; it's usually not far out of my reach.
Use good, heavyweight Cordura nylon for the exterior, with a lightweight water proof inner liner. Some bags have stowed plastic/nylon rain covers that work well.
Give the lid a quick access zipper for travel and a resin-based snap fitting for in-the-field use, so it's easy to open, but stays closed and keeps gear inside.
Sizing The Bag
A photographer who uses shorter lenses than I do will like the bag designed to take a medium sized DSLR, with a 24-875mm or similar lens mounted, and room for extra lenses, maybe two, plus a flash and some small accessories.
I normally carry an f/2.8 50-150mm lens. I'd also like room for three additional lenses, including a 70-200mm f/2.8, so I'd like a version sized for that use.
Add a couple of internal compartments and two external end pockets (flash, batteries, that sort of gear).
Reinforce the bottom with abrasion resistant outside, but soft surfaced so it won't scrape vehicle or other finished surfaces.
Internal mesh pockets for CF Cards / SD Cards, filters, cleaning cloths.
A comfortable shoulder pad, shaped correctly, is essential. It might help to make several pad shapes and thicknesses available.
Some photographers like loops to attach a medium sized tripod. That might be an option.
A cell phone pocket either permanently attached or snap-off, on the shoulder strap is handy.
I'm set to go with the above, if I ever find it.