A real contribution to the market would be frames that precisely fit glass plate negatives of various formats, especially 4x5. The plate is the glass in the frame. The modern size fashion has frames larger than negative formats. I like paper surround, too, when printing film negatives, but that doesn't work printing glass plates. I've managed to scourge a pretty good collection of old plate contact printing frames, and the technique that Jerevan linked to is a good one, but being able to buy well-made, new frames to fit the common sizes, plus Whole Plate and Half Plate would be great.
Also, I wholeheartedly second the motion for a frame style that doesn't punish arthritis.