It would be nice if we could add "Quick look"/"Lookup" function onto the middle button.
Gestures button for MX master should have an option to make it universal. Meaning, that you should have to commands there across ALL apps.
And if u like to make a custom button option if shouldn't delete the options but rather copy it and you should only change 1 or more options. Would be user friendlier instead of having to change verything.

Having a main framework to work upon for buttons would be great.

Most important: Quick loock middle button.