In architectural visualization it's a quite widespread convention to have vertical lines vertical. In SketchUp this can be achieved by switching to 2 point perspective. The camera elevation even stays at a horizontal after switching back to normal perspective. However, if you want to direct the camera slightly to the side you will also inevitably change the elevation and have to toggle two point perspective again to reset it.
I propose a modifier key to the Orbit and Look Around tools that locks the camera elevation to horizontal. This way you could rotate around objects or revolve the camera while keep their verticals vertical all the time. Pressing this key at any time when moving the mouse inside the tools would make the camera elevation "snap" to horizontal.
The same modifier key could be used in pan and zoom tool to lock the height/z-level of the camera. This would eliminate the need of the cumbersome Walk Tool when navigating inside building by introducing a simpler way to navigate with the eye height leveled.
I propose using Alt as modifier key since Ctrl is already used to suspend gravity in Orbit and Shift is used to pan in Orbit Tool.