I like what you have planned for the white areas. Any quilting you do in the brown diamond areas isn't going to show much, so I wouldn't bust a gut there. I'd do some straight lines, maybe quilting from corner to corner in the outermost brown squares, to make an on-point diamond, with other straight lines in the middle, to make a motif or whatever. The same with the brown borders---quilting there isn't going to show up, so nothing fancy there, either. You could follow any border stencil you like, I just wouldn't make it one that's overly detailed. In the white spaces around the motif you have already marked, I think more feathers would set off the circle well and tie in with the feathers in the center of it. You can find many stencils for feathered borders, to repeat four times around the four "sides" of the circular motif.
Edited: Oh, I see those brown diamond areas aren't made of squares, but rectangles. But you still can stitch straight lines corner to corner, etc. in them.