I've been kicking around the idea of making a G4, but I think the design, as is, is a bit too purist for me. Also, I keep hearing the same thing from people: It's huge... so huge that an ultralight load won't fill it up, unless you intentionally fill it with something (like maybe my inflatable Nemo 20R insulated sleeping pad). My thought is that if I was to build one, I'd add two primary features:
1) Better means of compression, like the version sold by Gossamer Gear, with compression cord loops between the side pockets and rear pocket... probably even more than they do, so you can get some serious criss-cross corset action going, and maybe installed under the mesh pockets so they don't get compressed and remain accessible. It'd lace up like a shoe. Pic of Gossamer Gear G4
2) An aluminum internal stay. I'd probably just pick up this one from Gossamer Gear
and tweak the bag design around making it fit. I've seen some UL pack designs that use sub-1-ounce carbon fiber rod setups, but for an additional 2.5 ounces, it'd be great to have it curve to my back. It seems to me that all it'd take is beefing up the top and bottom of the spandex mesh sleeping pad pockets with some cordura, and maybe installing blunt, ~1/2" diameter caps on the bottom ends of the stay so it is less likely to poke through fabric.
The other thing I'd like to do is make about a 3/4 scale G4 for my 4'6", 8-year-old son... then hang onto it for my 1-1/2 year-old son to use later. It'd be awesome to have a home-made heirloom like that, especially in this day and age of consumerism...