Looks great! I love all the bright/fun colors. for the quilting, you could do some ditching or outlining whichever you prefer around the colored areas to make them pop and then something a little denser (small stipples or matchsticks, or possibly a diamond pattern) in the long white areas to flatten them a little in relation to the colored areas. You also have a nice opportunity to do a motif in the white squares if you want, but it's not necessary.
if you have a sheet of plexiglass you could lay it over your quilt and draw designs with a dry erase marker to audition designs.
I'm a sit down quilter so unsure about threads on the long arm. On my dsm I would use a 100wt silk or Wonderfil in matching thread on the white areas if doing a dense stipple or matchstick design so the denser stitching becomes a texture that sets off the colored areas. You could use a 50wt matching thread if you decide not to go very dense in those areas, but still want to do something that will not compete with the colored areas.
I think I would either just ditch the colored areas or do a simple not very dense echo of their shapes with a thread that will blend there, so that it puffs a little more than the white areas.
Rob