I've thought about it, and I've come round to your "level breakage will have to be accepted" philosophy a bit more.
That's good. It opens the engine up to new ideas, and gives it more of a reason to exist. I'm also counting my own levels in this btw; many of my recent ones will not be compatible if I end up extending the laserer's distance cap, which is something I definitely want to look at doing.
as for the water, I'd say that it's technically just water with a skin on but it's up to you
I know what you mean, but it's actually the other way around: I'm suggesting that the orig_fire objects be assigned the effect of being 'fire'
rather than 'water', and that orig_marble, ohno_rock and (possibly) ohno_bubble be assigned the 'zombifier' or 'neutraliser' effect, so that any lemming entering these becomes a neutral or a zombie.
The fire one would be easy to do, since it's just a case of re-typing the object. "Poison water" turning swimmers into zombies/neutrals (probably favour neutrals slightly more here, incidentally) would require a bit more thought though, because they would no longer be straight-up 'water' objects, but neither would they be
only 'zombifier/neautraliser' objects; I'm not sure if an object can be both water
and something else, so... yeah, not sure exactly how this would work. In any case, I'd also consider doing this if NL gets the 'zombifier' or 'neutraliser' object type: at the moment, I'm not sure if it's getting these.
The 'ice water' idea is slightly more ambitious still, since it would require coding an entirely new object type (and, it's probably not worth it in all honesty!

).
But yes, you are right in pointing out that I'd essentially be re-using the water objects as "skins" for these new object types. All would still require some element of style re-designing, and in some cases extra bits of code.
How would you explain a swimmer turning into ice but a non-swimmer not doing so, though?
Hmm. Good point. Maybe all lems entering this water type should become ice, then. If a single non-resized ice water object is 64px across, perhaps the lems could become 8px-wide blocks of ice, so that it would cost 8 lems to cross the water. It's a bit of a silly idea, the more I think about it. I probably won't bother with this one.
Again, this is all talk anyway at this point. I'm concentrating on just the tasks outlined in the OP first. If I can get those done, maybe I'll look at other ideas as well.