I thought about some type of granny square.
But, despite how much I like the last one, I decided I wanted something that would stay all in one piece. Plus I still have to deal with all the color changes with granny squares.
I thought about doing a plain stitch pattern.
Or a pattern like these.
I really like that last one, and maybe I could adjust it to use fewer colors. I think it's definitely between the last two. Maybe I'll make one of each.