On the same REI trip that let me scope out the Sierra Designs Mobile Mummy a brief swing through the backpack section brought a new pack to my attention; the Exped Lightning.
The roll top can be closed either of the traditional roll top ways, either clipping to itself (the first picture) or to the pack sides (the picture below). An over-the-top-strap closes with a proprietary "g-hook" which I am also a fan of.
The real innovation is the cross stay at the top of the pack. Most single-stay packs I have seen couple it with a frame sheet to support the rest of the pack, this pack has a single aluminum stay perpendicular to the main stay that goes to each edge of the back panel. The pack kind of "hangs" off that top stay which prevents the top of the pack from barreling to the extreme. From the videos I've seen the middle of the pack body can still barrel a good bit but the whole pack actually does less so then a dual-stay pack with no frame sheet (such as my vintage Mountainsmith Frostfire). It also offers a solid platform to mount both the carry handle and the load-lifter buckles and offers quite a bit of articulation for the top of the pack as it pivots on the center stay.
The center stay does seem to lock solidly into the hip belt, which is mounted to HDPE wings that go behind the lumbar pad.
At first glance this pack nearly ticks all the boxes for my perfect light weight multiday pack: giant roll-top opening, clean exterior with the only external pockets being the dual water bottle pockets, absence of back padding and a good hipbelt. Upon closer inspection it does have some marks against it; the water bottle pockets are a little high for easy access without removing the pack, and worst of all is the compression system. The straps on each side are a single strap which adjusts at both top and bottom with ladderlock buckles, this means no quick release and lashing larger stuff like sleeping pads will be a tedious affair. Also rather then separating the pack into three sections (side, front, side like the Granite Gear packs) each compression strap goes clear to center front, meaning lashed items can wander around the pack body rather then staying put on either the side or front. This also excludes my favorite z-rest lashing spot, center front.
I will be playing around with the top cross-stay on some home made packs to see if I like it better then the frame sheet, only time will tell.