A whole cloth quilt.
I also like the idea of tone on tone white and then do a pieced quilt in a design (of which I'm drawing a blank on an idea for).
Since she wants it all white/white you then may want to ask what quilted design she may want on it. If you put a rose pattern on it, or hearts, or something she doesn't really like....
I would also think about putting their names and wedding date on it either on the top as a design element or at least on the back in the label for it.
Good luck and let us know what you decide.