Late to the party, but here are my thoughts:
Definitely do a smaller pack (make it narrower), and have an optional framesheet/stay. There's absolutely ZERO packs on the market that are low weight, low volume AND have a REAL optional framesheet/stay – and I see quite a LOT of people asking for this. These are folks who either don't like, or can't carry weight on their shoulders (mostly older folks) and want real transfer to the hips, but have sized down their gear for obvious reasons to near-SUL levels. They have two options, buy a heavy floppy large volume pack with proper weight transfer and underfill it, or buy a small volume frameless pack with no real weight transfer and often a too-short torso length. Both of these options suck! This is a great untapped market. Just look at how many MYOG projects there are for this product (small volume but with a frame, or a small volume frameless adapted for a MYOG frame)!
I emphasize REAL because there are optional framesheets and stays, but with the lower weight UL packets currently on the market (i.e. Zpacks), they do two things VERY wrong:
1) do not contour to the back, so they tend to sag away from the back, which puts the weight back on the shoulders, significantly reducing the proper transfer, plus it makes the pack extremely unstable (bounces around)
2) do not have a solid connection to the hipbelt for proper transfer
To do it right, you either want a 'ceneterish'-attached hipbelt, or a tall/narrow pack, otherwise the hipbelt won't function properly. If the base of the pack is wide, and the hipbelt is attached to the sides of the pack, like almost all UL packs, then the hipbelt has very little gripping power and will slide down, significantly reducing it's usefulness (and annoying most users). I think this is why so many people prefer webbing belts on UL packs – the way it's designed means a real hipbelt doesn't really do much.
I'd add an optional removable lid, similar to MLD, as well as an optional removable flap (instead of a lid). This will keep the simple drawcord closure, as well as provide the 'waterproofing' similar to a rolltop for people that want that feature. Make sure the lid and flap contour to the upper pack's shape properly (i.e. NOT like MLD). See Zimmerbuilt's Zen pack for proper lid/flap contour and upper pack coverage.
I like your pack design, and I'd offer it with three options:
1) hipbelt
a) a real hipbelt that is attached 'centerish' a framesheet/stay pocket for the optional framesheet/stay
b) webbing hipbelt attached at the sides of the pack with small wings (MLD Burn)
Make sure the real hipbelt is hipbelt pocket compatible and that the connection is secure (i.e. Zpacks).
2) main bag size – have two patterns, one wide/deep, one narrow/thin, both about the same height, so you can accommodate multiple torso lengths, this allows you to have different volume packs, offer a 25L main bag size, and a 40L main bag size
3) front pocket
a) no front pocket and daisy chains down each side (climber's pack)
b) front pocket with 4 small loops on each side for shock cord connection (backpacker's pack)
The daisy chains will be bomber for climbers to hold tools and other gear. And the 4 small loops (ala MLD Burn) on each side of the pocket will allow casual backpackers to attach shockcord/regular cord to for their own lightweight ice-axe attachment, CCF pad attachment, fast rain gear storage, top lid/flap attachment, or other random attachments.
I'd also design the shoulder strap attachment such that you can attach the shoulder strap anywhere along the torso length to make the torso length completely customizable. This is the MOST important custom feature, IMO, to getting a proper fitting pack.
For framesheet/stay design, I'd use choroplast with aluminum rods, a heat-formed HPDE framesheet, or a GossamerGear-like hoop stay with a pocket for a CCF pad (GG sitlight?) that is attached to the hoop stay sleeves (so it's not pushed outwards by the load). I'd consider making the pocket/sleeve external to the pack so that it can be removed easily without messing with the contents of the pack. For padding against the choroplast or HPDE framesheet, I'd have a second sleeve in the pocket for a CCF pad – make it loose so a sitlight could be used, or a double-layer laminated CCF could be used (with extra padding on either side of the spine for a tight, stable fit).