Provide a guidance on a standard way to replace complex menubars.
There is a gaping hole in the current Gnome HIG: it provides no standard UI model for applications with complex menus, Gimp and Libre Office. While the HIG does state that complex apps can continue to use standard menubars, Gnome's CSD initiative (HIG and Wayland architecture) is pushing app developers in the direction of abandoning them. The HIG itself is contradictory on this score, with one part telling developers that headerbars are recommended for "all applications" and another part telling them that menubars are fine in some cases.
Aside from Builder, all feature-rich desktop applications have menubars. Currently, nobody has a clue about what might take their place. Headerbars and contextual menus obviously can't hold all the features - and contextual menus are a big challenge to design and implement.
Gnome pressures developers of these apps to abandon the menubars that most users rely on, yet neither Gnome nor anyone else has proposed an alternative. LibreOffice is considering giving Gnome users a ribbin interface on Wayland, just to comply with CSD. Yet this ribbon interface is not well-liked by users and doesn't even correspond to any interface model by Gnome HIG. So it's inferior in terms of usability and it's inferior in terms of integration with Gnome. Yet that's what we going to get, simply because it's one way to get rid of the titlebar.
In my opinion, this is extremely dangerous and the consequences will be felt once CSD propagates through the ecosystem. Unless something is done, the UX/UIs for the most critical applications will go in a dozen different (and possibly misguided) directions, with no hope of ever picking up the pieces. Right now they all have menubars and some are wondering about redesign - so now is the only time to issue a guideline on replacing menubars.
The header-bar is physically incapable of holding lots of features. One very basic idea is to have large command palette (another screen effectively) with command search and clickable items as a supplement to the headerbar. This gives you a big canvas to put features in, without obscuring the content. But this is just a one idea. (Ideally, the headerbar and whatever interface we choose to replace the menubar should be made available as a library, so that non-GTK apps can use it without reinventing the wheel every time. But that's another story.)
I therefore urge for the Gnome design team to finally address this festering problem and create a UI model that can replace the menubar in complex applications.
The UI model should be reasonably simple and uniform, so that apps don't need in-house design teams to implement it. The headerbar, for example, is a simple drop-in replacement for simple menubars and toolbars. This allows for wide adoption.