1. Solo, Mute, Bypass - public variables or methods to set them.
2. Ability to access Effects like Attenuation. Remove exposed parameters - why even need them.
Setting by string when you can cache the effect in variable and access it's values directly? Why not?
3. Ability to add those Effects through code.
4. Ability to add groups through code.
5. Ability to add snapshots through code.
1,2,3 are priority. They are essential for creating good audio system.
Right now the way to mute, or play solo is to disable all of the audiosources and add them in code in the beginning to access them later, or have a value that multiplies the volume by some scale. But I guess you have this multiplication value internally if it's possible to mute like that in editor, or change volume in Attenuation.