Something that would be neat is some various ways in which a single creature can individually control many bodies or separate body parts, all at once. Even though these ideas are kind of separate, they're similar enough that I probably shouldn't clutter up the suggestions forum with multiple posts:
Severed body partsOne thing that would be neat, is for some creatures to be able to, after having a body part removed, keep controlling it remotely; like, they lose their arm, and it crawls around and acts according to the intent of the main body/soul (as opposed to current undead body parts, which are basically "new" undead), or if their eye comes out, they can still see with the eye no matter how far away it is.
Of course, this will have limits in accordance with the body part; a toe can't really move by itself or do anything except sense touch, so they will probably just be lying there. Naturally, such a creature would not have any need for nerves to control their bodies, and probably not much else by the way of requiring air or sustenance either.
Such creatures may be able to fix themselves by reattaching the body parts, either by mundane means like stitching them back on, or being able to just pop them back in place by hand and regenerate the connection on the spot.
This is a common trope for undead to have (zombies in some media, and almost always skeletons), but would be cool for other magical beings as well, or people cursed with true immortality for knocking over statues in a temple for example.
Naturally separable body partsParticularly for automatons and elementals, something you often see is creatures that are made of multiple objects, like a stone elemental whose arms are levitating rocks floating beside its main body for example. It differs from the above mostly in that this is not the result of injury; they
naturally have body parts not connected to other parts, or which can be removed at a whim.
These usually can't or don't move their body parts long distances, but some creatures with such a capability may be able to send body parts on long missions. Also unlike the above, a "removed body part" is usually able to move itself around with little difficulty, often levitating.
Vermin swarmsIt should be possible for creatures to have bodies made of swarms of vermin, like rats, or worms!
Firstly, I will point out that for obvious reasons, they would not be simulated individually, because you'd need literally hundreds of thousands of them and every computer would melt.
Now then, these could work in a variety of ways, and may vary from world to world depending on mythgen, and maybe even have multiple systems in one world sometimes:
A1. One "central object" that has the soul in it, and if you destroy this core, the swarm disperses/dies. This may be a magical artifact, a normal individual outside the swarm controlling it, or maybe a specific individual in the swarm. If it's a member of the swarm, they may be able to swap which one has the soul in it.
A2. Distributed; that is, if even one part of the swarm lives, all it needs to do is breed more, or find new members to join it, and you're back where you started.
Creatures may either:
B1. Naturally
be the swarm; maybe this is a forgotten beast that exists only as a possessed mass of flesh eating beetles.
B2. Temporarily transformed into a swarm, like a vampire turning into a swarm of bats. If this is an A2 creature, then if some but not all are killed, presumably when they transform back they will have some form of injury, proportionate to how much of them were killed. Such creatures may be able to have a special kind of "dodging" that involves turning into a swarm just before being hit so they can move out of the way.
B3. As mentioned in A1, they may just be an external individual/wizard or whatever controlling them using magic (or maybe they are a beast that has these creatures always surrounding it, obeying its will).
As for abilities, all swarm creatures would innately be quite amorphous; that is, you can't chop a leg off a giant bear made of flies, because most of the flies will dodge/shift out of the way, and if they can't dodge (maybe it's not flies, but worms), then you'll kill some and scatter the others, but the parts that formed the "leg" can just reform back into that shape! If they are a B2 creature, even if they lose an arm as a vampire, may be able to turn their arm into bats along with themselves and reform fully.
Most of these creatures presumably would be able to incorporate normal vermin into their swarm, like an ant monster that's been depleted finding a wild ant colony and "possessing" them, and may grow larger over time by "breeding" within themselves, and presumably if you split off/scatter some from the "main swarm", they will still form into a "sub-swarm" and try to re-merge with the main group (or if they are an intelligent/villainous being, perhaps they could direct parts of themselves to different locations to fulfill different agendas), though some less powerful beings may need to be all in close proximity (if they are A1 type for example).
They would also generally be able to move through tight spaces (anything their respective vermin could pass though), and could attack both by the usual "bite or sting" they may be able to do, or they could "engulf" dwarves to suffocate or crush them, or both. (these abilities would strangely enough also have a lot in common with this suggestion for creatures made of non-solid materials:
http://www.bay12forums.com/smf/index.php?topic=178483.0).
Some swarms could be "shapeshifters"/formless blobs that take specific shapes only if they need to (for example, as in the image above, maybe they want to disguise themselves), or may always have a specific form; this might vary from creature to creature.
Multiple full bodiesThis is pretty much the same sort of thing as the above idea about vermin swarms; basically, a being that is a single soul controlling many bodies; but they wouldn't all be piled together physically on top of each other, they would be far more spread out, and probably smaller in number (though they might possibly be very large/be entire "civilizations" of one soul controlling thousands of bodies).
Normal beings that were not born/created with this sort of power presumably would have some coordination issues with using more than 1 body.