If you've created a giant ship with a large wing on one side and it takes a couple of hours to build that said wing. You would know that having to recreate the exact wing again but for the other side would be a real pain, right? So my simple suggestion is for a build mode to do exactly that, select a specific section of an area designated by the user, copy that selection and mirror it on to the other side of the ship, to then be placed with a click of the mouse like a normal block selection. If anyones played minecraft with the admin tools allowing the ability to select a large chunk of an area and copy it or undo that copy, you would understand what I am attempting to suggest.