@Tim Edwards I have been thinking about using overlay cells, and magic forbidding them for extraction. I think for 1-1 overlays (one cell overlaying another one) just having 2 variants of the cell would be an easy solution. But for example IO cells, where you might want a basic IO cell with several different overlays for the digital logic (input/output/bidirectional), and several different overlays for different ESD capacity strength or some other options, there a modular overlay option could make a lot of sense, and having to generate each possible combination of overlays would get unnecessarily complex. Just thinking ...

What do you mean by "magic forbidding them for extraction"? I use overlay cells all the time.

You said in the video D2SK1_L4 that magic does not support to spread devices across different cells. So overlay cells must not change the behaviour of cells.

That only means you can't split a single device across subcells, like having poly in once cell and diffusion in another. There are plenty of ways to do overlays with all the devices (whole) underneath, and the overlays just connecting them together in different ways.