Thanks for the aclaramient Clément.
From what I have seemed to appreciate the overlapping faces of the same object are sorted by identification number and by material. The materials under the pile are painted at the end (above).

It is difficult to see in the second image, it shows how some Backfaces are painted erroneously above the faces oriented to camera.
I hang another image where it is better appreciated. The yellow and green grid material are the same. There are two objects to the left and one with two materials to the right. From above the transparency is seen correctly, from below the order of painting is reversed.

Just a mockup of Property Editor more responsive to the area width.
It involves vertically dividing the editor and setting the visibility of the panels located on the left partition.
Here an animated .gif

I think for lights and cameras it would be wise to keep an iconic shape with fixed size.
I also believe that the current shape of the lights could be differentiated more, better define the direction of light and show its intensity. It would be useful to be able to edit some parameters by entering Edit Mode or a new Mode where the basic parameters of the objects are visually editable. I painted white shapes that could grow or change color, depending on the color of light or intensity.
I have added an iconic shape to the camera, I have removed part of the hidden mesh of the pyramid and reduced the size of the triangle that marks the vertical.
Combining white and black for the shapes makes them more visible.

I think Alt 3 is correct although compared to the initial icons, the outline should be brown, not black or gray.
Anyway I added a couple of proposals adjusted to the style of the icons of Blender.
My vote is for Alt 5 :)

For me the last alternative you have outlined is good, although it is radial the circle is not closed and can be interpreted as general. I leave some ideas, I personally like the arm, shape is complex but the silhouette is very recognizable, on the other hand it is generic and because it is also in WhatsApp the association is also fast. The boxing glove looks like a cloud.

I've been playing around with the shader and wire of the objects and I upload some of my thoughts for Viewport representations.

The customization of the color of the wireframe by object go against to the color symbology used by Blender, for this reason I have been exploring a bit with the contours of the active selection to see if it is possible to separate the function of color and the customization.

I have made a gif where the new selections are highlighted for a few moments.
I do not know if it can be useful or distracts visually.
If this don't slow down the workflow, I think it can help to better orient yourself in the scene, especially when jumping from one object to another in sculpt and paint mode, or when you have two overlapping models, in high and low definition version, and you don't know which one you are selecting.

Suggestion:
The new grid loses opacity as it approaches the clippig / end, since the horizon is parallel to the view I think that the opacity of the grid can also fade as the angle of the plane and the angle of view is smaller.
When the position of the camera approaches Z = 0 the grid should fade.

Opacity of the edges relative to the length of these on the screen.

What if, in addition to filtering the visibility of objects with the collection, are also highlighted or filtered objects with labels?

Wow, the grid is cool! You might charge less if you gain opacity near the selected object and updates while object is moving.
The last suggestion of today. Perhaps objects that can not be selected should be darker or cooler.

Show optionally, proportional transformation with an editable color gradient

The mesh subdivided could lose opacity according to the number of subdivision.
The UV islands may have different colors in Edit Mode.

A couple of small suggestions:
When you want to move an object in Z and Y is pressed G + (Shift + X), I think it would be more comfortable to use G + X + X.
With Ctrl + LMB, selections are made with lasso, it would be more useful to do it with Shift + LMB, adding + Alt to execute the rectangular selection and + Ctrl to invert the selection function.

Re images #2 & 3: I guess you mean being able to select a transform manipulator axis, so that the next time you LMB-drag the transformation will be constrained to the selected axis? I thought about this idea too - it might be pretty handy - but I'm afraid it will be annoying for people who switch back and forth between using manipulators and shortcuts/LMB-drag: they'd always have to make sure no axis is selected if they want unconstrained transformation using LMB-drag. I yet have to find a proper solution to this.

There are keyboard shortcuts that only work in one area and one mode, and there are others that always act, such as F12 to render or Ctrl + S to save.
I do not know if it would be a good idea to ban the general shortcuts and make all specific for the area and working mode. In other words, Ctrl + s should only act on the information area, for example.
I only comment as a reflection, as I think this might facilitate the editing and maintenance of the shortcuts.

Hello,
By default Blender accepts the actions with the left button and cancels with the right. Some addons like Mira accept with the right button, and others like Craver sometimes do it with Space.

Ohh yes Thomas, for me it's the perfect solution. With "Active layer" could be possible to preserve the old Blender's funtionalities, and overwrite objects representation.
I would add the possibility to filter objects's visibility by object's name or anything else (subtractively).
I guess the partitions are intended to render layers, not for the 3D viewer, isn't it?
Here a fast mockup.

Without giving much importance to the 3D cursor, we can have the next interaction with the 3D manipulator, leaving the right mouse button to select exclusively.

Currently Blender allow to have an object in several layers, the mock up of
Paweł Łyczkowski, maintains this possibility. Is an extension of the current model and looks great.
If the objects were unique for a layer, the next a mock up would make sense.

Except that the layers override substractibamente visibility and selection, I do not see that differ too much from the groups. I believe that the layers should also allow overwrite smoothing, and wireframe visibility, and other object properties.

The mock up is very cool, this functionality it will be a big improvement for Blender.
Maya has recently improved its layers rendering, I recommend that you take a look at their news.
https://www.youtube.com/watch?v=AZu-zjtd6PQ

I have thought to put the slots on top of tools menu, and add more slots, and add edit mode version, but ... forget it. It has a conflict with modal tool options, and becomes too confused.
Most of the time we need to configure the tools and not replace them, although the slots could be filled with presets, it's too crazy.
Maybe a good representation and easy configuration of mouse shortcuts it's enough.
Thanks for your attention, Zbrush possessed me.

I see a relation of this last post with the firt one, where there was some slots for tools. Instead of calling directly a tool I suggest to call a slot, more or less like Zbrush does.
I think it's something more than a key configuration.
(Context Sensitivity + Modes + Presets)

Thinking in your words Jonathan Williamson, I have thought maybe would be a good idea split key comands in "key shortcuts" and "mouse shortcuts". This way would be more visual to see how user interacts with modes (and tools?), and change the behaviour if neeted.
As I have said in other thread, in my opinion spaceBar should be in this table too.
As we can see in this table there is a lot of empy space.

Before organizing Blender in task oriented environments, I think it is worth trying other modes of organization.
In this Mockup I simply added a tool shelf in property editor and I have removed it from 3D view tools panel, this way brush properties goes with other properties, such as texture properties, multiresolution deformer...
I added three slots for active pinzeles, the second and third is accessed by pressing Shift or Alt. All slots are modifiable. Ctrl inverts brush function same way it does now.
Perhaps with more slots we could assign more keyboard combinations.

Other similar option.

Although things are going in another direction, I make this entirely different proposition.
I believe that the SPACE bar is underused, it's a long key that lets slip your hand around the keyboard without losing sight of the key. We could use it to do combos with other keys in the same way that Ctrl or Alt.
"B" Key (= Blender) could replace the current role of space, "B" it is in the center of the keyboard too, and it's accessible with both hands.

Pies + sticky keys seems a good combo, this way we have direct actions and menus in one key. Anyway I think using pie menus without sticky keys and QWEASDZXC for accessing to pies actions would be faster. For instance: W for opening Transform Pie and Q to active Translation gizmo. Never mind, pies and sticky keys seems good option too.
On the other hand, I would use C for positioning 3D cursor, and holding i t for opening snapping object and cursor menu. It would leave left mouse click for other purposes.
Long time ago, I mixed Nathan's experimental hotkeys with standard hotkeys and I got and interesting behavior:

·1 rotate camera + 2 holding Alt -> snap view
·1 holding Alt +  2 rotate camera -> Set camera preset (front, top ...)

For Pie Menus, based in pie piece distribution and left hand's position , I think sould be better to use QWEASDZXC short keys instead 789456123 (or both).

Assuming the default camera can not be deleted and clicking twice the "Z" key can temporarily access their attributes

"For the display and shading panel, that's impossible to add all these buttons in the header shading's mode selector"
Why not? All these parameters could be inside camera attributes.

I would move all attributes from all view properties to regular properties window.

Personally, I prefer the previous version

more exemples