Would love to be able to adjust HSV, brightness and contrast (with preview) while converting to indexed mode.
Sometimes mapping a RGB image to a palette is not optimal. It would be great to be able to adjust the image (HSV, brightness, contrast) with a real-time preview while converting to indexed color mode.
My suggestion is basically to add the current HSV panel and brightness/contrast panel (including ability to pick channels) to the “Color Mode > More Options : Advanced Options” panel - like this mockup, just way better 
