So after quilting in the ditch around all of the appliqued pieces, I decided to kind of echo quilt the rest of the centre panel. There was no measuring involved - I just used my eye, so it's certainly not a uniform pattern of quilting - just as the appliqued pieces are not uniform.
Now for the first border. I am thinking about freehand cross hatching. Freehand because I think anything too uniform will conflict with the design.
I have also read Caroline Friedlander's book these hols - Savour Each Stich. It's a good read. I like that it's got lots of words in it. Words about pattern, design, lines, contrast, etc. It is worth looking up.