I agree with JohnDempsey, I'd like the option to Show hidden depenendent objects, as well as dependencies.
However, a work around for now is to remove all objects except your entry point. From there you can add parent and child objects to diagram (Right-click -> Add to Project...).
Its not ideal, especially with a large diagram (mine is over 3k objects), but you can slowly step through your subsystem.
Also, Show/Hide still allows you to search for hidden objects and the diagram does not have to redraw when you show stuff. Adding objects requires the diagram to redraw, which can still take some time even with the very fast diagram types.