In making levels, there's many, many times when I've had to stop building, place a few lemmings down and then load the level and try things to see if miner tunnels line up correctly with where I'm expecting them to come up, if builder bridges can reach over gaps I want them to reach over, whether or not lemmings will bonk their head or not while they're building, and so on. It'd really great if there was a way to gauge these sorts of things in the editor without having to load them game up and try them out manually.
So I propose the addition of some rulers to the editor. A splat ruler, a builder ruler, a miner builder, and so on. For the builder bridge, I'm imagining a semi-transparent representation of a completed bridge which you can just move around and line up however you want (either as a fake terrain piece, or bound to the cursor), with some indication of the height at which the builder would bonk their head on the ceiling too, if possible. For a miner, it'd not only show the 2:1 descent, but also the 'width' of the tunnel, so you can gauge how much terrain will be removed. Things like that.
Would there be interest?