In the Instances window, it would be great if the Interpolation numbers could be incremented by ones with arrow up/down or by tens with shift-arrow up/down, with instant feedback in the sample on the bottom, the way that many other number input boxes work in Glyphs.
Also, being able to customize the “Ang126” sample text would be useful.
For now, you could just select the instance in the preview area and then open the instances tab. You should see the changes to the instance in the preview.