titanium rod, the hard kind, like you use in your stakes, would be cool, different thicknesses. 0.08" for small projects, and something around 0.12" or so for stays, not sure, 3mm, 4mm something like that, but the grade 5, I used lower grade stuff on a recent project and it's really soft.
I've been thinking of using that for a backpack frame along with some other stuff.
Also, carbon fiber tubes if you can source them reasonably, goodwinds is expensive and it's a gamble if what you order is going to work right for a pack, I'd rather get stuff like that from you, could trust it's strong enough and appropriate wall thicknesses and so on. Tap plastics here has some variety but it's very limited, though you can get some very cheap stuff from them that works ok.
Something to think about, the xpac vx07 stuff in usable colors, that's an interesting material, maybe a touch light for durable packs, but interesting to work with, and almost nobody carries the color ranges, maybe it's just not available from dp. Not sure you'd want to compete with most of the online fabric places, except for orders of materials sure to sell, like vx07. I've been tempted to get a big roll of that and sell the rests, but if you do it, no need.
I'd avoid competing too much with diygearsupply, he's been doing a good job in his niche, very cheap and decent quality stuff, though his new shipping charges might hurt him a bit.
Note that buckles vary widely in quality, the ones that zimmerbuilt is selling are really nice, very smooth, and although a bit heaver than the cheap stuff from other suppliers, nice. The difference between buckles is quite amazing, something I realized too late when I realized I'd accidentally used two different types on my new pack, didn't realize they were different, looked similar. So closeup pics of the items so you can really see would be helpful.
He sells the cord adjusters/side release buckles by the way, but they are very expensive compared to other side release buckles, small market I guess and special order.
Oh, and those needles people recommend but that nobody seems to ever have in actual stock, what are they, schmetz something or other, not the universal ones, microtex or something?