I've been exploring caves with adventurers mostly, and I've noticed that there is a significant lack of slime-based creatures. We have mudmen, and there's apparently grimelings as well, but neither the classic nonsentient ball of goop that eats whatever it comes across even if such things are alive nor any of the variants seem to exist. Thus I propose the following:
* Normal slimes. These guys eat anything they find, and if one of your dwarves accidentally bumps into one they might try and eat him too. Weapons can get stuck in them, and if the fighter is really unfortunate can be completely enveloped, in which case (depending on balance, coding, and such) the weapon either becomes lost and the slime gains a level of size, or the weapon slowly degrades inside the slime's body, and only once it's completely destroyed does the slime gain a level of size (during which time you have the opportunity to try and kill the slime another way, and perhaps retrieve your weapon). Their blood is paralytic, which means that if your fighter is truly down on his luck he will get paralyzed and end up being eaten by the slime. The larger variants (whether gelatinous cubes or simply large slimes) will be able to completely envelop your fighter, whereas the smaller variants will only be able to envelop part of the fighter. Any bodypart enveloped will suffer damage until it is completely destroyed, at which time the slime will grow a size or two (depending on how large it is already and how much it actually ate) and move on to the rest of the prone fighter. If the fighter gets completely enveloped, their whole body will take damage until he/she dies, after which the slime/cube will proceed to crush and absorb the body of the fighter (as well as grow according to the ideas mentioned above). I figure these types of creatures have to have a fair bit of control over their material structure, so they will also be able to absorb blows to a certain extent. Another idea I heard on #bay12games was to make it so that if you have too much miasma around your fortress that a slime comes around looking for stuff to eat.
* Acidic slimes. These do not paralyse, but they will corrode your weapons with each strike as well as your armor and flesh with each hit (it's only a chance for a bit of the goo to land on you, but if it does... it'll eat a hole in your armor, and if it touches your skin, you'll be looking at exposed bone pretty soon). They won't have any ability to block, though, and can be fairly safely killed from range; barring this the quality of the metal used determines how easily and quickly it corrodes (copper will melt away quickly, but adamantium will stand up quite well to the acid).
There's certainly more variations possible (fire/lava, metal, etc), but for now I think that should do for comments.