The only time I have allowed a customer design a quilt, she was here with me and she got the fabrics she wanted, and selected a pattern from a magazine. When I was done she had the quilt she wanted. Otherwise it becomes a headache, since her vision may not be your vision. Do you intend to have them send you fabrics to make it as well as backing and batting? Yes, even the batting matters, because they may have in mind a heavier quilt, or a softer one. The quilting design have to be discussed as well. Many people don't know what they want and those are the hardest ones to please. I had a woman who asked me for a quilt using a wolf panel in the center. When I was done she was happy, but when she went home she wanted to return it because it was a few inches shorter than she wanted. She asked for a quilt for queen size bed and that is exactly what I made. She was expecting for the quilt to be almost to the floor all the way around. I did not return her $ or took the quilt back, because by measurements I should her that she got what she ordered, but I know she will never ask me to make another quilt.
To avoid all those details I like to just make a quilt and sell it as it is. I have done quilting for someone else's finished top, but she told me exactly what she wanted. I finish my quilts and post them on my web site, or Etsy.