After lots of tinkering with Quilt-Pro, I came up with this possible layout, done in grey scale. I scrapped the alternate block plan, and instead framed the original blocks, using an idea from Sharyn Craig's Great Sets, and set them diagonally. When the frame is colored in two values, this three- dimensional effect appears. (I can't remove the patch lines, so the effect might be stronger if you squint so they disappear.)
This is all very tentative. I don't want to overpower the original blocks, and this design is quite strong. Also, I have no idea about the colors, and even more importantly, I don't know whether it would require only two fabrics to be sure the values stayed consistent, or whether I could go scrappy. Scrappy would probably lose some of the 3-D effect as values changed. Would that be good or bad? Also would the framing pieces need to be two values of one color throughout, blue for instance, or could each X design be a different color? Since the original block is multicolor, I'm thinking one color, but I'm not sure.
Part of the decision is related to available fabric. If I keep the same fabrics throughout, I'll have to buy fabric. I don't want to, because I already have too much. A dilemma!
So I guess I'm at the auditioning fabric stage. It's fun to think out loud, so to speak. Thanks for reading.