The trick to adding really large borders to get quilts a size you want them without having the quilt end up looking like that is what was done is to incorporate the border into the quilt either with piecing, applique or quilting. I really like Madquilter's idea of doing some vine and flower applique in the fabrics you used in the quilt. It would definitely add to the quilt's beauty and be in keeping with the style. If you are a confident quilter, I like doing extra wide borders to use as a canvas for my quilting. But I have never gone as wide as 15". Anyway you may wish to consider leaving some of your extra wide border just plain cream instead of the 4" round if you want to do some wonderful quilting. Here are links to examples I have done:
This border was 12" wide
[ATTACH=CONFIG]543642[/ATTACH]
On this border I combined the two into one with the quilting. I think the two combined measured at least 12" but I believe it was more:
[ATTACH=CONFIG]543643[/ATTACH]
On this quilt I modified the pattern and made the border to measure 9" instead of the 4" so I could basically "quilt an applique design
[ATTACH=CONFIG]543644[/ATTACH]
And my most recent quilt the quilting I had planned most definitely dictated the border size.
[ATTACH=CONFIG]543645[/ATTACH]