New suggestion.
Yet another random idea off top of my head. Might not work too spectacularly well but interesting to consider.
Weak terrain.
Basically this comes off of a strange and rather cruel idea that briefly flashed in my head for no apparent reason.
I was imagining how a basher tries to bash thru the base of a gigantic hill but then the earth collapsed on him.
And so born the idea of weak terrain. There are a number of variations with this, but the basic idea is it's an invisible "field" you can assign to the terrain, analogous to metal areas and one-way walls.
Weakened terrain has the property that during excavation, with a short bit of delay the column(s) of pixels starting from the point where you removed terrain, will fall down until it hits ground.
So for example, say we are bashing, and the weak terrain pixels are marked as "X":
XXXXXXXXXXXXX
XXXXXXXXXXXXX
XXXXXXXXXXXXX
XXXXXXXXXXX
XXXXXXXXXXX
XXXXXXXXXXXXX
(I know a basher's tunnel is taller than that, just humor me ok)
Then after a short delay, preferably once the basher bashed further and out of the way or something, the terrain will fall down at some unspecified speed and eventually the terrain becomes:
XXXXXXXXXXX
XXXXXXXXXXX
XXXXXXXXXXXXX
XXXXXXXXXXXXX
XXXXXXXXXXXXX
XXXXXXXXXXXXX
Although not completely decided, I'd say the lemmings who gets hit on the head by such falling columns of terrain pixels will be killed instantly, although to make it sensible there probably should be a minimum height and falling distance for the column to be lethal when falling down (having a single pixel knock the air off a lemming is a bit silly even for this wacky idea).
I'd imagine the terrain probably falls at the same speed of a non-floater lemming, maybe a bit faster. I also imagine that the columns of weak pixels affected, during the "delay" phase when they're about to fall, they'd flash so the player is aware of the imminent danger/problem. For extra punch, when the falling columns of pixels finally touch ground, maybe we can draw some confetti or dust cloud or something. And of course we'd need some good sound effects.
I haven't decided yet on whether cascading terrain collapse is allowed, but I think it's a neat idea on top of a simple collapse. Cascading terrain collapse could occur if an entire column of weak terrain sit in mid-air unsupported by normal terrain below. For example, say we have a bridge-like thing with water or air below:
XXXXXXXXXXXXXXXXXX --
XXXXXXXXXXXXXXXX |---- all weak
XXXXXXXXXXXXXXXXXXXXXX --
XXXX
XXXX (water or air here)
Then after the first collapse we get:
XXXXXXXXXXXXXXXX --
XXXXXXXXXXXXXXXXXX |---- all weak
XXXXXXXXXXXXXXXXXXXXXX --
XXXX
XXXX (water or air here)
If we allow cascading collapse, then since the first collapse lands on a column of completely unsupported weak terrain, that column will be triggered to collapse after another delay, and so we finally get:
XXXXXXXXXXXXXXXX --
XXXXXXXXXXXXXXXX |---- all weak
XXXX XXXXXXXXXXXXXXXX --
XXXX
XXXX (water or air here)
-----------------------------
The interesting property about weak terrain is that it is more difficult to create a path through such terrain by excavation, because the collapse will quickly close the path off. At least until you excavate enough times so there are no more terrain to be collapsed from above, or unless you started off excavating so high that there is nothing to collapse from above. At the same time, this collapsing property can be useful in its own right: imagine demolishing a tall column blocking the crowd's way by bashing thru it, and imagine it sits on entirely weak terrain and we have cascading collapse enabled. Then after bashing, the column will eventually fall off on its own. If that column was originally in the way of a second higher platform, you have effectively clear out the obstacle with just one basher, instead of two (bashing at lower platform and bash at higher platform) you'd be required normally.