First off, sorry if this belongs somewhere else. Also, I know I'm prolly a bit new here to be posting suggestions but I think this is a pretty legitimate one.
Basically, you could use the blueprint system to create a custom block that would be its own block type, similar to the object tool. The main difference being that you just plop it down like any other block and it snaps to the block your reticle is on, just like any other block. I understand that this is kind of possible right now with the blueprint tool (make something within 4x4 and then use the snap tools) but the time taken to achieve this could be severely downsized with a separate tool imho.
This sounded a bit better in my head, but I think I got the point across