That is a strikingly beautiful quilt as is. I wouldn't put any sashings in it. If you need to add something to increase the size, then add it to the borders. Add a narrow yellow border, then a wider black to finish.
I see there are 20 or so suggestions from 20 or so people. :lol: It's all up to you now! Please post pictures of this when you are finished. It is lovely.