To lengthen a project or when I have to change fabrics either because I've run out or because I want to make a statement, I've used variations on what I call the Zipper. You take two fabrics, typically used in the top already or close enough. Sew a strip and cut into rectangles and then into a long pieced 4-patch. You then sew a strip on either side of the four-patch unit.
You can be using pieces to go in and out of the Zipper, that is scraps for one side and a consistent fabric for the others. You can take cornerstones from sashing and lead them out to the borders. When you are out of one fabric, you use the consistent one next to what you already have and then add the close/different fabric on the outside so it doesn't touch so much... All sorts of ways to play with them.
With 2.5" cut strips, it adds 8" each side, or you can do all sorts of modifications from no side borders to smaller ones.