Good luck with this
I like metal buckles etc. because they don't fail. I like plastic ones because they can still be undone with cold hands when the straps are soaking. I'd suggest using both plastic and stainless as needs must? Maybe stainless for the luggage to bike and (replacable without cutting them off) plastic to secure rain flaps etc?
Making them a size/shape to fit pannier loops is a good plan. Touratech, Hepco and Becker and Metal Mule loops are so similar it'll make no odds to your design if that helps. You need the bag to bag straps to be long enough to go over a bike with frames fitted, that's where a lot of soft luggage I've tried has failed, they assumed seat width only.
The stuff I've made for myself I like to give straps so they can be used as flight/hotel/backpacking luggage too. I try and avoid extra straps that get in the way, rather I sew on extra fastenings to convert from bike mode to walking mode. No daft little plastic wheels in the bottom though please
I'd ask that the bottle holders be designed round pop bottle type containers as well as Sigg bottles? Beer and whisky bottles then fit by default
Can we have colour coded buckles or something? I get hacked off when I load the panniers in the tent, place the wrong way round on the bike and spend all day looking in the left side for what's in the right. Green buckles on the right, red on the left?
Can we have the means to secure excess lengths of strap? I hate trailing bits but won't cut them off as the next bike is bound to be wider/taller/fatter somewhere.
Can't wait to see the results.
Edit to add: IMHO Price is a tough one. I havn't bought anything for years except MOD castoffs. IMHO every single item in the TT catalogue is overpriced full stop, M&P stuff is overpriced for what you get. I'd pay £100 for a really top class pair of panniers with maybe an extra 30 for the bullet proof lining thing. Above this and I'll modify Army bags. Now I don't think I'm your key market, I think you can pitch between M&P and TT prices so long as the quality and design is off the scale. Once that actor or his mate use a pair, then you can charge £500 a set. I think you need to very very carefully pick the right RTW rider and give a pair away where you'll get some positive feedback here, on ADVrider, on GS club etc. Look how Metal Mule took off, five years ago they were undercutting and outperforming TT. Now you pay for what you get (and then some).