On the matter of integrating related projects more closely together, do check out this how-to, which may be of use to you.
In a sense there already is a “third bookmark type”, in that every item in every project has a dedicated link that can be used to open the project and navigate to the item with a single click. This link really works anywhere, not just from within Scrivener, but since Bookmarks are also a way of storing such URLs, file links and so on—it works there too, and quite handily.
A workaround in the meantime is to create a “global” project which I can then just keep open in a separate window, but a more streamlined, built-in way of doing it would be preferable and cleaner.
So yes, it is an improvement of that basic idea, but you’ll still have that idea. Honestly I don’t know if this could get much closer than that though. At least not without some kind of radical overhaul to the entirety of Scrivener’s way of working.
The main problem is that one project cannot edit another closed project directly. Such would be mayhem (with how it is designed), and so the best it can do is passively link to its resources. (We probably could make a passive read-only view, but I suspect that would only please some, and it wouldn’t offer much over having the project open.)
And is that really in the end much of a problem? Perhaps I don’t understand the core resistance to having more than one project open at once, because I usually have more like half a dozen open.