This idea would allow for significantly faster construction of buildings and other multi level scenery creations, as well as fixing disorientation due to the game's isometric perspective.
Essentially I would like to be able to set a 'work plane' to be able to build scenery onto. Right now holding shift and dragging the mouse up or down will show a purple 11x11 tile grid that allows you to align your scenery at a certain height with other objects easily. With a work plane a player could press ALT or some other hotkey while holding shift at the desired height to lock this purple grid into place. The player could then place multiple objects at this same height without needing to re drag from the ground for every object. Objects would be placed as if the purple grid is the ground. Pressing shift again would remove the locked work plane and would return to the normal placement system.
If the hotkey system isnt possible for whatever reason a simple ui popout (like the cutaway view ui) could be made to set the work plane, perhaps even with options to show the purple grid across the entire terrain.