That is so sweet. I have a feeling you can't go wrong here, whatever you pick will probably be loved!
I like the idea of a Bargello - if you use her favorite color and your favorite color that might make for interesting symbolism within the quilt - the two colors fading towards each other to make something beautiful! They look much harder to make than they really are, it's really just a bunch of strip piecing.
And then when you quilt it, you could maybe hide things like hearts or your names or your wedding year in the stitches. Or maybe other symbols and things that have special meaning to the two of you.
Gee, I think *I* am getting ideas! Next year is our 20th....hmmmm...