The Container would be a new plug in. It would open as a blank resizable window, similar to a control surface. But rather than adding UI items, you could add any tabbed window from VDMX.
When adding a tabbed window into a container, the window would not take on the size of the container, allowing you to arrange multiple windows of different sizes into the container.
The benefit of the Container is that it would have its own local preset for controlling items inside the container.
Basically adding a mid level of presets for when you want to change many settings with one click but don't necessarily want to use Workspace Preset.
Containers would also be tabbed windows so you can nest multiple container into the same container window and switch between them using the bottom tabs.