I was already planning have some kind of cabinet above the toilet. I have that now. I'll also definitely have a medicine cabinet above the sink.
Point taken about storage in a vanity. The problem is that I loathe getting down on my hands & knees to fish things out of the vanity. Invariably, the only accessible items are toilet bowl cleaner, shower/tile cleaner, glass cleaner, and a sponge. I want easy access to things. What I'm considering is some kind of wall-mounted vanity with a pull-out drawer and a vessel sink on top.
This is a Kohler "Purist". Something like this.
With a shallow closet between the bath and the guest bedroom, I'm stealing space from the guest bedroom that doesn't change the furniture layout in the guest bedroom. I'm only thinking of grabbing 4" using 2x8 framing instead of 2x4 framing on that wall. On the guest bedroom side, I pick up a built-in bookcase behind the shower part of the wall. 4 linear feet of shallow closet in the bath gives me a ton of storage space in a cottage that is really lacking it.
Corning-Pittsburgh makes Energystar-qualified glass block. You can order it pre-assembled in a vinyl frame. That's what I was planning to use. It won't be as energy efficient as a 2x4 wall with insulation but I need the natural light. In researching windows in the shower, glass block kept popping up as the solution least likely to leak.
The only plumbing I have anywhere near an outside wall is the kitchen sink. The plumbing comes up into the cabinet from below so it's not in the wall. I have a forced hot water kickboard radiator with an electric fan underneath the sink.
The wall mounted tank has a plate for the flush mechanism. If you remove the plate, you can get at everything to replace any parts of the flush mechanism that have worn out. If something really goes wrong, it will keep the tile guy employed afterwards. I want to tile high enough behind the toilet so it's easy to clean if the wall gets pissed on. In a 5x8 bathroom, I really want to pick up the extra floor space of a wall-mounted toilet even if I'm risking a PITA tile repair job every time something serious happens with the toilet.
Note to self: Have a few boxes of spare tile on hand.
I have a crawl space underneath the bathroom floor. The floor is coming up anyways to install radiant heat. I can do whatever I want to accommodate a shower pan or shower base. I'll take a look into terrazzo as an option to cast iron or tile.