I think your idea is a great one. We did something similar for my daughter's wedding last summer. She chose to use the wedding colors in the quilt. Together we made 9 patch-blocks. Those are set with plain blocks in pale version of the colors. On the plain blocks we have, since the wedding, appliqued photos from the wedding. Guests signed the white squares in the 9-patch blocks. The couple enjoy looking at the quilt and remembering the fun they had at the wedding and all who were there. (We chose a 9-patch because my daughter was also doing the sewing, and is not as experienced a quilter. It was a fun memory-building time.)
If you want to use a more complicated pattern, why not choose a signature block of some kind?
Do you have a chance to talk with the bride-to-be as to colors? If you are close, perhaps you could offer her the choice of patterns, as well.
They look like a very happy couple. Congratulations!