Is your purpose in this wallhanging to practice different fillers? I ask because some duplication behind the 4 corner flowers would bring unity to the piece. I hate to tell you this because it is already marked, but I would change the fill you have 2nd from the top in the middle. The density is so different from the others that it looks odd to my eye. Either put more fill in that one with the zig zags or use the zig zag filler to go behind every flower in the corners and use a different fill that is more dense, like your others in that space. I recently posted a sneak peek of diamond quilting with fills here:
Sneak peek of quilting with high contrast thread
Most of your other designs appear grid based in the other squares so maybe stick with another grid based design there instead.
Judi Madsen of Green Fairy quilts does fabulous ruler fills behind applique that look great. She has pictures on her blog, instagram and pinterest. Just google images of either Judi Madsen quilting or Green Fairy and you should get loads of ideas.
Another quilter that does great grid based, dense fills is Margaret Solomon Gunn. You can do the same to see images of her stuff. Here is a pinterest search result on grid based design fills to give you some fodder for ideas.
https://www.pinterest.com/search/pin...yped&term_meta[]=grid%7Ctyped&term_meta[]=based%7Ctyped&term_meta[]=quilting%7Ctyped&term_meta[]=fills%7Ctyped
Google zentangle images, lots of fill ideas there too.