Scene groups would be awesome, especially if they could be organized like tool icon drop-downs in toolbars.
On the Layers issue, I would like to group my Layers, but my enthusiasm for this feature comes with a serious concern: Adding the ability to group "Layers" could encourage people to think (even more than already) that the arguably-misnamed SU "Layers" are truly Layers, and newbies especially would therefore more likely try to organize their model using the Layer panel, instead of using SU's "Layers" as they really function, which is more like an unsorted list of Hiding Tags that get applied to non-sticky objects to facilitate batch-hiding. It already can take years for people to learn that organizing an SU model happens using Components and Groups, and the hierarchy can be easily viewed in Outliner (at least, if people named their objects and organized them nicely). Outliner already shows how the hiding functions of Layers are inherited. Grouping Layers would probably have to carefully avoid any hint of inheritance, by not being able to batch-hide the layer-group all at once. What I mean is that the Layer-group couldn't itself be a Layer. The image made by @sketchuparchive could be dangerous.
The only advantage I see in having Layer groups is to avoid scrolling to find the Layer, like what we're wishing for in Scenes. Layers is already a vertical list, though, and the scrolling isn't that bad.
My suggestion for an immediate solution for Layers: When I want to "group" layers, I simply create a number system. Adding a number, 0-9, to the beginning of the Layer name and then sorting the Layer names alphabetically creates a grouping effect, organizationally. If a Component Instance has a Layer just for that one Component Instance, and there are other Layers solely for sub-components, then I name the Layer for the top Component in a way that it will appear alphabetically immediately on top of the Layers for the Subcomponents. That keeps them all together on the list.