For me it's a large main compartment, a large lid pocket with double zip pulls and stretch fabric so it can expand if full. add a front pocket with two strips of stretchy material either side of a strip of non-stretchy fabric and a compression strap that goes round the top and to both sides. what this does is give a large pocket for wet gear like bivvy, tarp or shell layers. The vertical stretchy strips would allow gear to be held in the bulges in this fabric without movement. the non-stretch centre is where the single centre lid strap goes. I like angled side pockets big enough for bottles made out of stretchy fabric with drawcords to cinch closed around contents such as bottle. angled for bottle removal and replacement without stopping to take bag off. I'm now a convert to hip belt pockets, very useful. I'd like pockets on shoulder straps too but they must be removable and easily put back again if needed. In fact I think the shoulder straps should form a series of fixing points for anything from phone pockets, emergency whistles, compass, other pockets and a large map/gear pocket that stretches across whole front like OMM does. I like the little plastic bits on innov8 racepac for attaching chest straps,whistle and various optional extras like pockets.
Back systems need to be simple and work. I like lightweight kit so it needs to be beloiw 1kg if 30-40 litres or about 1.0-1.3kg for 40-50 and even to 60 litres. My preference not to everyone's liking of course.
fabric needs to be light but tough. waterproof too but no taped seams, perhaps welded if dream pack. Also not rough so it won't wear hard shell jackets on contact points.
internal frame which is removable and made of alloy strips linked in a pivot at shoulders. only for larger sack sizes of course. small volume sacks don't need a frame IMHO.
The above is my dream, wish list.
reality for me was a modified Aiguille Alpine sack. they're very good at making custom mods. just find one or two of their standard sacks as start point then discuss your wish list of design changes. they'll let you know what works and that they can do. got a good, long back length sack that was light,ntough, simple and made with my personal choice of fabrics. tough where needed, lighter in low wear areas.
