I think this is a hard quilt to plan.
It all depends...Do you want to soften the edges?
Then I would go with a plan mentioned above for a feather or a fern in the colored squares...continuous curve in the little black squares and then maybe a meander or loops in the white.
If it were me....I think I would maybe keep with the continuous curves in the black squares...then in each varying color come up with a different design..in the top layer some pebbles, the next layer...curvy back and fourth lines..then swirlies...the last one curvy plaid lines. Something like that??
Then in the back ground I would do something fun like loops and loops within loops....or Charisma Curls..or something really curvy. ......if you didn't like all the varied designs in the squares you could just do wavy lines all the down going back and fourth to make those linear squares just stand out against the curvy back ground design.
But That is just my opinion.