Being that you're wanting to increase the size of the project, how about some sashing and cornerstones? Take a look at the countless other 4PPs that have been shown for some ideas as to how this can be accomplished.
Also, for your border, the original fabric, could work well, if you should have enough.
ITA with the My2Pals and your DH, that the border blocks you have shown use are not the right colour match. I'd look at black for your main sash/border colour along with some batiks. The 4PPs are what you want to see, and not any bordering you create. It should be a frame, not a focal point.
Keep it simple and don't take the eye away from the beauty of what you already have.