Is there a way to make a new primitive Scene?

(Note: If it makes a difference, I'm a Mac developer.)

In Swift UI 1, you create your application View instances by piecing together various Apple-supplied views. If they weren't sufficient, you could create new primitives with a hosting controller supporting a UIView/NSView setup.

Is there a similar method for making new custom primitive Scene instances? What I read about the Apple-supplied ones so far don't cover:
  1. Making a mode-less dialog panel, which will have at most one instance. (If you activate the command for the panel when an instance already exists, the current instance is brought to the foreground, instead of making a second instance.)

  2. Making a setup similar to the Document system, but the source data are URL/URI/URNs instead of FileWrappers (i.e. the each pseudo-document is an internet download). A web browser would work like this.

  3. The ability of a window being made from a command off another window, and you want at least one of the two windows to keep a (weak) reference token to the other.

Can this capability be added before the Fall releases if it isn't there already?