I didn't suggest to build every building diagonal, like you are picturing it. The whole thing could be limited: In my picture the inclined castle part is connected to a vertical part just fine.

Look at the castle here. The author did work around the vertical parts to connect several unconnected castle parts which were not build in a perfect line.
With inclined buildings, the alchemy labour would be inclined to the elevator - and the castles would connect.

But yeah, I see your point as well, example: Inside a mine, there is no room for a vertical building, but diagonal the building would fit into it. Because of that the player will build a leaning building ontop of a vertical underground - that should be avoided.

>Sooner or later the landscape will change and the "natural" leaning of the buildings will look really silly. And when buildings start adjusting and rolling around, all bets are off

Hm, well I don't know. A small adjustment would be: the higher diagonal side would come down to the smaller if: No materials around the buildings are holding it, nor another building is doing that job.

[Edit]
I'm not sure if diagonal remains in a worn out landscape really look that bad, I'm still all for trying it out :)