This suggestion is actually copied from Clones.
In Clones, they had this type of interactive object that looks like a sort of short tunnel, with one end larger than the other end. Clones that entered from the small end will emerge enlarged from the large end, and vice versa.
The real point is not so much this object per se but the idea of having a "size" state for Lemmings. Lemmings can be either normal size as they are now, or be "supersize". Like Clones, there are some important behavioral differences with size, the most obvious being that when you have supersize lemmings excavate, their actions would take out proportionally more terrain (bigger explosions, taller bash tunnels, etc.) then their regular-size counterparts.
So here's the proposal:
1) introduce a few new interactive object types. One type would convert a lemming passing over it to supersize, another to normal size, and yet another that switches sizes.
2) have two sizes of exits, a normal size and a supersize exit, to be explained below
3) have two sizes of entrances, a normal size and a supersize entrance, so that it is possible for lemmings to enter into a level starting off supersized
4) Here are the behavioral differences between normal and supersize lemmings:
i) excavation and building are proportionally larger in effect. Basically, for exacation you would proportionally enlarge the masks used to take out terrain pixels, and for building you would proportionally enlarge the build bricks, with the net effect of a supersized build being longer and higher.
ii) supersized lemmings moves more slowly than normal size lemmings, when walking and climbing
iii) supersized lemmings falls and floats faster than normal size lemmings. Optionally, supersized lemmings may also have a shorter max-fall distance.
iv) the min-wall height (the minimum height for a lemming to turn around at a wall instead of walking straight up it) for supersized lemmings is proportionally larger.
v) supersized lemmings may only exit the level via supersized exits (they can't fit thru the normal size exits). However, normal size lemmings may exit via either sizes of exits.
vi) this is optional but a potentially interesting behavior: supersized blockers only affect supersized lemmings, and normal blockers only affect normal sized lemmings. The "rationale": the normal sized lemmings could just walk between the legs of the supersized blockers, while the supersized lemmings could just step over the normal sized lemmings.
That's about it for the differences, I can't think of anything else that should be different between the two.
I don't have a clear idea on what the actual size differences should be between the two.
