I've got a Panzerfiets which is designed to carry Ortliebs. HPVelotechnick do make custom aero bags that neatly fill the space between the rear rack and the back of the seat, which give you a bit more volume than the standard back-rollers, but I've never seen the need and prefer the bags to be usable on my other bikes.
If I wanted to improve on the typical Ortlieb, I'd combine the internal pocket of the Roller Classic with the simpler closure of the City[1], and possibly use the lighter weight (if less durable) Plus material. Then I'd improve the lower hook attachment so it stays put while bouncing around off-road. Otherwise, it's a simple, proven approach. Boxy is good when it comes to filling it with stuff.
At the aero end of the spectrum I've got an 80 litre GRP tailbox on the Red Baron. This has the same sort of capacity as a full set of touring luggage[2] and actually makes the bike go faster (carbon would save a few kilos and be faster still). Other than the ability to lock it closed, I remain unconvinced by the merits of an easily scratched rigid structure for day-to-day use, and while I don't mind cycles making reasonable mechanical noise, the sounding board effect grows tiresome. I think if you're going down that road for touring, you might as well go the whole hog and use a Quattrovelo or something. That said, it is strangely liberating to be completely unconcerned about the *volume* of what you're carrying. You can just throw jackets and things in there loose.
[1] What's the point in using the shoulder strap to close all four panniers? You've only got two shoulders, and probably can't carry the combined weight more than about 50m without the aid of a bike. And if you carry the strap outside the bag, it gets covered in muck. I carry one shoulder strap internally for when I want to keep the front pannier with the valuables with me, and close them in the drybag style, which is a faff.
[2] Unfortunately the bike is not designed to carry the weight of a full set of touring luggage, and handling becomes a bit iffy if you load it heavily.