That was exactly my thought too...in scrappy colors.

If you used the piano keys border you would create a 'heavy' feel to the outside of your quilt that otherwise has a 'light' feeling because of the use of so much white. If, on the other hand, you used the idea toverly suggested you would continue the 'light' appearance. I think I would make a small section of each idea and audition them. But, please don't forget to show us what you choose. Your quilt is very pretty.

If you still have plenty of scraps, you could add an appliqued vine out of them. Or perhaps small basket blocks which would echo your on-point squares?

Depending on how much bigger you want it - I would take off the border you have now and add two more rows of blocks. Then put a border back, or not.

My first thoughts were the same as Theodora and JustAbitCrazy. If you are concerned about that being heavy, use many more very light fabrics or put white in a lot or even every other one. Piano keys can be any length (width?) so you can make it the size you need.

I love your quilt! I like the idea of removing your on-point squares and just adding another row of your center blocks all around, then reattaching your squares border. It should have another border of white on the outside the same width as you have on the inside of it. Then use a brightly-colored binding to finish it all off. It's a beautiful quilt and will be worth the trouble.

This is where designer software like EQ really comes in handy. It's going to be a pretty quilt when finished.

I might add a moderate border of white so that the diamonds have something on their outside. Someone else said a scrappy binding would add flavor. I love what you have so far, by the way.