Hey funbun,
My perfect pannier is a lightweight pannier, that is easy to take off the bike, expandable, mostly weatherrpoof and solid in construction. My perfect pannier are the panniers on Backpacking.net as mentioned earlier... I posted the directions.
About marketing lightweight panniers. One of the big reason I posted the directions online was to feel how much of a demand that there was for my design. In the year plus since I've posted the directions, I've had only one person email me for help and get a few mentions here and there. Realistically, the demand for lightweight panniers (or just my design) is not high. As noticed in this post, most people want more pockets and an easy to remove, rock solid attachment, which means more weight. If you want to make panniers then you will have to decide between designing a pannier for the masses, which won't be light weight and will have to compete with a saturated market, or make your own perfect lightweight pannier, which won't be that marketable.
I'm glad that you have graduated from making bucket panniers to sewing some. Its a much harder process and much more rewarding in the end.
Just my thoughts from someone who did a similar thing,
scott