Much of the photography I do involves being very active on a trail. I do a lot of hiking and low mountaineering and when I'm out, especially in the winter, it's difficult to document the trip without falling behind or holding everyone up. In addition to this, I need to carry other gear with my photo stuff. If I could have the perfect bag, it would have room in an upper compartment for dry clothes, layers I've shed on the trail, a little food, an extra liter of water, a map, compass, small first aid kit. This is actually very realistic for a day pack. A smaller, lower compartment would be padded (especially on bottom) and have room for an extra body, wide angle and normal primes, and a small, slightly telephoto zoom lens, plus accessories like extra cards, batteries, lens cloth/cleaner, polarizing filter(s). This pouch would be front loading to allow for ease of access from behind or on the ground. Plus side pockets capable of holding various equipment, like possibly a lens or skinny water bottle.
The shoulder straps would include reinforcing to accommodate heavier loads (a weakness in many packs, unfortunately), a small chest strap to keep the shoulder straps on, and, more uniquely, reinforced heavy duty loops (preferably metal/plastic) along the length so that I could hook a body and water bottles on them, along with an extra pouch to keep an extra lens in. I'm not talking one or two loops, I'm thinking more in terms of a bandolier for the adaptability.
The loops would allow me to have a secure place to keep the camera when I wasn't holding it. Often when on a trail I need to use two hands to navigate, and the camera just flops around my neck. This is annoying and the strap is too long for when I'm not totally upright. The loops would allow me to keep the camera closer to my body when scrambling and to keep it close to snapping shots on the fly and swapping lenses on the fly via the detachable pouch.
LowePro has a few models that are very similar to what I like, but most, if not all, dedicate most of the pack to camera gear. Photo gear slots are essential, but there are other items to take into consideration that are must-haves when out in the wilderness and weight is a factor, hence the lack of a desire for room for a giant telephoto connected to a body. That would be great on a trail, but realistically they are too heavy when the photography is secondary to the destination.
Ha ha I know this is a tall order. I in no way expect you to take this as a request for a custom bag or want you to think that this is what you will need. These are merely considerations I take into account that may or may not possibly serve as food for thought for you or anyone else that reads this.