Somehow the download is a PDF. Honestly, I think this would be awesome with feathers in the white areas, but if you aren't comfortable doing FMQ, feathers won't be good as they can be challenging to do. If you are up to sending out, I really think that would compliment the quilting beautifully. Charisma here on the board does amazing feathers free hand.
If you definitely want to do it yourself, then how about replicating some of the shapes of the colors in the white areas? Hmm, that sounds confusing. Give me a minute to play with the picture to give you an idea ...
Here it is as a picture:
See the green outline of a block on the left side of the quilt. What if you replicated that shape to help fill in the white area? (Sorry, I couldn't get my lines to go straight in PowerPoint).