Allow for the menubar to be placed in the server-side decorations (locally integrated menu)
Something like this as a per-app opt-in feature, minus the buttons:
This idea has been discussed before in various places and it's generally popular.
One of the gripes with CSD as currently implemented is that it totally breaks consistency between applications. If one wants to create an application that even remotely fits into the modern Gnome DE, one must write it in GTK and use CSD. This creates unnecessary barriers to entry for traditional apps (i.e. most apps).
A few arguments have been made against this idea:
The first is that it goes against developer wishes. However, I have yet to see any developers complain about people using global menu and HUD-search with their apps. And the global menu is not designed as an opt-in feature: it doesn't work well unless it transforms the UI of every application on the deskop. A local menubar would be much less invasive - the UI won't change too much, and not all apps need to support it.
Many programs explicitly support menu export over dbus - so that their users have access to stuff like global menu and HUD - but their developers cannot offer this feature to Gnome users because there is no support for it in Gnome shell (third-party or otherwise). Gnome users can't even use HUD menu search, because the appmenu implementation in QT is designed to swallow up the menubar, which means that you must draw it somewhere outside of the application.
Furthermore, many developers would like to save vertical space and create apps that integrate better into Gnome, without having to rewrite their apps from the ground up in GTK with CSD support. For example, VSCode recently stopped providing CSD by default on Linux because it just didn't look native at all, which upset users. The point of their CSD was to provide a colored titlebar with a menubar. If they could use Mutter's native decoration to provide these two simple features, they would have no problem. So they actually want to do it, but are restricted by the DE. And needless to say, the lack of feature also goes against the wishes of many users. So by not providing this functionality, we'd be actually restricting choice for both developers and users.
The second objection, if I understand it correctly, is that server-side decorations should just disappear from Linux. However, there is no chance of this happening and most non-GTK developers would be staunchly opposed to this idea. It seems like strange argument. Finally, some people have argued that the UI should be totally encapsulated in the app's window, regardless of what developers or users want. I never understood the rationale for this, especially since Gnome had an appmenu for many years and nothing bad happened. Furthermore a developer could write a Gnome extension (or tray icon) for their app, thus breaking the principle of encapsulation. For example, Gnome's own media players will show some controls in the Gnome tray-menu on the right-hand side. Should they be prevented from doing that?
Alternately, Mutter developers could make it possible to add arbitrary widgets to the window decorations. Currently, the server-side decorations are the only part of the DE that is totally outside the scope of extensions. However, doing it properly might involve more work than just providing locally integrated menus. So this is just an idea.
A related benefit may be better support for scaling. Some apps may fail to properly scale, rendering their menubar inoperable. If the menubar is exported and drawn using GTK, then the app may be at least partly usable.