A few comments:
1) Like 75% of this post implies features that don't exist yet:
-Reinforcement implies digging
-"Air source" implies ... something. carbon dioxide poisoning?
-Torches imply lighting
-fire buckets implies firefighting and significantly different fire mechanics to allow firefighting to every be feasible
-"insulation" implies a much more advanced temperature model, also would only be relevant on the surface even in real life, since rock is about as insulating as you can get already.
This is almost impossible to comment on or to make suggestions of criticisms, because if the main feature doesn't exist yet, we don't even know if these suggestions would be relevant to it, let alone the most efficient. As just an example, if digging could only happen horizontally (unlikely), then plates might make sense, but if it happens in 3D, then you'd either have to completely plate every wall and every floor everywhere or it is pointless, cause they'll just reroute through unreinforced walls. If so, the feature seems not very useful, since 99% of people can't afford that much plating. It's very hard to say without knowing how digging works, which is why I suggest waiting until it is implemented, or at least in a changelog, "coming up" state first.
2) Blood would be brown as a wall pigment (nitpick)
3) I like the decorations idea, though, the one that is currently implementable. Just for raising value and flavor.
4) Engraving is not directional currently. Yes, Toady could code something to make it directional, which would matter for torches, probably. But for decorations alone, I don't see it as necessary. For example, you've been playing the game for however long, and you didn't seem to notice that engravings are not directional, right? So you probably wouldn't have noticed decorations not being directional, either. Also, for most things like "studding a wall with iron" or whatever, it would make sense to divide the decorations around all sides, just like engravings make sense, you know? Not for totems, but whatever.