I really like the ideas by @UAS-Design to even show “sub-objects” in the forthcoming Object Manager. For now we would be happy to just get something where I can select objects NOT in the viewport. Supposedly the new Block Manager panel was the ground-work for it, but @wim has recently stated that it is not targeted even for Rhino 9.
I will probably be retired before we see any object manager in Rhino, let alone anything as good as suggested by UAS-Design. Don’t forget we had an Object Manager in Rhino 3 and that was released in November 2002, so 23 years ago. If it takes that long again, I will literally be 65 and about to retire.