It is as simple as it sounds: why just don't add solid settings which now are within a separate window ("Layer - Solid settings" or Ctrl/Cmd+Shift+Y) into corresponding layer's dropdown? It would be a lot simpler to access solid's color and size properties as well as keyframe them and add an ability to make gradient fill like it is in Photoshop. And of course, this feature will have a positive impact on the workflow of any user from a beginner to a professional.