In reply to tulka shaman.

So… Why not just “update” these old themes, upgrade them to become block themes… With ab automatic migration mechanism where it is needed…

I like this idea. It’s a fun exercise to build old themes directly in the site editor. I did it with Twenty Fifteen for fun, but I thought about doing it for all of the old default themes back to the original one.

Sidebar navigation is possible, but there definitely needs to be more design tools for navigation blocks, especially submenu options.