Considering how much even a 10-mph headwind sucks (because that’s a 10 mph wind + 10 mph cycling speed = 20 mph of wind resistance), how about aerodynamic panniers? In bicycle racing, you can’t add accessories only for their aerodynamic effect, but touring isn’t racing. I’m imagining semi-hard-sided plastic panniers instead of nylon ones. They’d be completely watertight and could be vastly less drag – I’d estimate a Cd of 0.3 would be doable versus rectangular nylon bags with flaps and zippers must have Cd’s near 1.
I’m thinking of something like a kitty-litter pannier, but tear-drop in cross-section. Those empty kitty-litter containers are (well, the first one I grabbed out of my stash is) 11 ounces, 300 grams, and has a usable volume of 7″ x 11″ x 11″ = 3.7 gallons = 14 liters (for free). Or, for a pair:
28 liters, 22 ounces and still free.
A quick check of REI’s offerings finds (all specs for the pair):
22 liters, 33 ounces, $90;
30 liters, 59 ounces, $220;
40 liters, 67 ounces, $190;
42 liters, 78 ounces, $250;
So even if you doubled the weight of the original kitty-liter jug, it would compare favorably to commercial offerings. With a shroud added in front and fairings added behind (also from thin HPDE so it would be heat-welded and/or pop-riveted together) it could have a shape something like this (original container in blue, fairings in black):

which would also increase the useable volume by 50%.