The Ethereal Prison

The Ethereal Plane as written in D&D is a transitive plane. It's a place you travel through or the medium stuff floats in. What if it were a bit less accessible--on purpose? What if, like the Phantom Zone is DC Comics, it was a prison? Maybe the gods or super-wizards of ages past had imprisoned renegades, criminals and monsters there?

If you aren't familiar with the Phantom Zone, read about here, The concepts a pretty simple one, though, even if you've never heard of it. Imprisoned creatures float around like ghosts.

This would have the advantage of differentiating the ethereal more from the other transitive planes and establish some interesting mysteries for PCs to look into.