Block height tool
Here is a little though? When building up to a high point it get's pretty annoying as i have to keep clicking the z axis button to toggle up and down to reach under an existing block. Then keep flicking back to check if it's the right height or even go to auto. I had this trouble on making my tower as the huge symbol at the back is over 240 blocks tall.
So i thought that holding right click would enable the z axis so you could freely move your mouse and the use the scroll wheel (or +/- for those who don't have a scroll wheel) to move the block or item up or down. then press left click to place the block at a precise place. If you or someone let go of right click then it would stay at the height you/they let go of the right click button. This could save time when building something which could be more effective and easier to use.
The original controls would still be available for those who might prefer the old way.
David Alan Gay commented
2D Shadowing would help as well.
Love this idea