.... and what do you want to do? It could become a passthrough or, you could seal it up, and create a tile accent... You could have custom stained glass, so that you still get light, but have privacy between the rooms...

I was just in a kitchen that had mirrored doors there with shallow shelves behind. Standing at the sink didn't feel like facing a blank wall because I could see the whole room behind me. And the shelves were handy location for stuff used in that part of the kitchen. It was a very elegant installation, retro-fit by an architect for herself. Maybe that would work for your situation.

Glass shelves?

The mirror is a good idea. What would be fun is, if you could get the mirror custom made to include the stained glass piece you already have.

Is there a reason you don't want the pass through to the new room?