'Contrast' comes in many flavors, and that selection of fabrics has more than you are giving it credit for. It has contrast of scale, value and texture just as it is and your pattern choice adds contrast of shape and size. All together, the voice is understated and restrained, yet solid, strong, and reliable. My sense is that is what your gran was instinctively drawn to. Think about her personality before you decide to add "oomph" or any loud contrast of color.
Placement of the fabrics within the pattern will be very important to the impact, so try several arrangements before you start to cut and sew anything. I think it has the potential of being a stunning, and very personal, quilt for your 'favorite human on earth'. Enjoy the journey.
