Personally, I love the large lampwork buttons that are hand made by artists for something like this. Since their source for glass is the same as what's used for the beads you used in the stitching finding one that would compliment your stitching shouldn't be difficult. With the size of a lot of the handmade buttons, I don't think you'd need to worry about more than one button, unless you just wanted to have the buttons fade in and blend in so that your stitching is the primary focus. Personally I love to find a large focus piece (in the same way that the face of a watch is the focus piece) to compliment my work where the two form a harmonious piece that is greater than the sum of its parts. But that's just a me thing.
Hope I've not made your decisions more difficult.