I had this same problem with my little sister, my family has always been huge into quilts because of our grandmother so when I asked her what she wanted, I feared the worst and got it - she wanted a Double Wedding Ring.
To be honest, I haven't started it yet, the design became bigger than me, she went with fall colors for her wedding and I was up to 24 different colors, not counting the background color, which I chose. I scrapped my original plan to do it and I'm now focusing on designing a Diamond Double Wedding Ring using only four colors. The weight of the world is off my shoulders.
I would think something timeless and classic would be good. Grandmother's Flower Garden would be good and actually looks quite romantic in neutral/flesh tone/subdued colors, Shar Jorgensen done a wonderful show on it years ago that allows you to machine piece it instead of hand piece it and right now, I'm working on a lap quilt that seems to be morphing into a queen-size quilt, you can find the show on YouTube, just look for Quilting from The Heartland.
A log cabin design would be super fast and super easy, as well and done in the wedding colors would look fantastic, I've always loved those designs. I would suggest to just find a pattern that you enjoy, then add the wedding colors, which tend to lean towards the couple's favorite colors anyway.