This quilt is so beautiful and visually pleasing. I would leave this quilt alone and just make another one that was larger. I think that adding to it will take away from the symmetry. Personally, I don't think that any of the ideas given would do it justice. You do what you think you need to do but I think you will be disappointed although I hope not.
FYI, this is one of the prettiest quilts I have seen on this board and I have looked at lots of them.
PPS I think maybe ube quilting's suggestion might be okay. Notice I said might
