Here is the initial design. EDIT: -OUTDATED-, see posts below for newer one/ones.
Collections are supposed to take on the current role of Layers in Blender - this is indicated here with the general look of the Collection Manager. Meaning that they take the familiar look of layers like they are usually displayed in graphical software, while staying consistent with the Blender’s Outliner look:
When a Render Layer is set to use the Master Collection (could be named ‘Show All Collections’ in the UI), the Collection Manager changes its GUI a bit to reflect that: