Since we have a currently-ongoing debate about changing the appearance of one-way arrows, now seems like an appropriate time to bring this up:
While trying to fix backroutes to my levels, I repeatedly ran into the issue that there is no type of one-way arrow that allows a Miner to pass, but not a Digger - and vice versa. This pretty much solely boils down to the fact that horizontal one-way arrows can still be dug into (as predicated by the original Lemmings rules, which shouldn't be altered), whereas downward arrows are more strict, reliably preventing Bashers from passing through them. In other words, horizontal arrows only point towards the one side from which you
cannot remove the terrain, whereas vertical arrows (both downward and upward) indicate the direction in which
you have to move with the destructive skill.
Now, since downward arrows have been around for ages, I don't think their behaviour should be altered either - thus, it would still be impossible to create terrain that allows Diggers to pass, but not Miners.
However, what we could create is a type of one-way arrow that allows Miners to pass, but not Diggers.
You guessed it: I'm talking about diagonal one-way arrows! In order to make these more universally useful, I'd make them a little less limited, with the
arrows actually pointing in both directions, but at an angle. Moving diagonally along this trajectory would be possible, whereas moving along the opposite diagonal line would not be. Straight vertical or horizontal movement through this terrain would of course also be impossible.
As such, diagonal arrows would allow both Miners and Fencers to pass, but only from opposing sides - e.g. a Fencer from the bottom left or a Miner from the top right, but not a Fencer from the right or a Miner from the left. And then vice versa for diagonal arrows in the opposite direction.
Now, obviously, there are many cases in which you want terrain to be Miner-friendly, but not Fencer-friendly - or vice versa. This is usually one of the most critical steps to enforce whether lemmings can pass through a chunk of terrain only from above or only from below.
And this takes us to the question whether several different types of one-way arrows can be stacked. (I haven't actually tried this yet.)
If this is the case, then you could already create unidirectional Miner terrain right now, by stacking downward arrows with horizontal arrows: The downward arrows would prevent bashing and fencing, while the horizontal arrows would force the Miner into one direction only. Diggers could still pass through both unaffected.
Now, if the same logic were applied to diagonal arrows, you could indeed create Miner- and Fencer-exclusive terrain by stacking vertical arrows with diagonal ones: The diagonal arrows would limit the terrain to being removed by either Fencers or Miners only - and now, depending on whether you have upward or downward arrows stacked onto them, you could limit the area to only Miners or only Fencers specifically.
In general, I think the ability of Diggers to bypass any type of one-way arrows, except explicitly for upward ones, is a massive loophole that can easily be exploited for backroutes, independent of the setup of the individual level.
Therefore, we could really use a tool to rein in the Digger.