Oh my ... what absolutely lovely applique!!! So neat, so precise, so uniform!! BRAVO!!
I'm thinking perhaps quilt a snowflake (perhaps 1/4 the size of your flakes) in the area where the 4 squares join (and a half flake if only two squares join on the edges), and fill the remainder with micro stipling ... or some other micro stitching (loops, Mctavishing, etc). This will fill the bulk of the negative space with something other than just background (ie stipling) but it will still make your applique the star of the show. In some places where the space between applique pieces is minimal, just echo or run a single stitch between each to make each section pop.
This is assuming you were not planning on sashing.