Usually come up with an idea and then buy fabric.
Lately, though, I've been working on what my hubby calls "leftover" quilts. I'm into long-straight borders with mitered corners so much that I buy the extra yardage even if the border is only 1" wide. So I have all this fabric "leftover" that's no longer 45" wide and not too great for strip piecing with other fabrics. So I've been making pinwheels. I'm actually ready to construct the top. Only needed 4 colors counting the white. I'll post pics soon. Bought fabric for borders, but plan on using the whole width.
Have 2 more "leftover" quilts in the planning stage. It's hard to come up with something that requires the amount of fabric you have, just in case you can't find it anymore.