So, if we just summarize what we now know about the 6 cosmic forces, the picture will be: each of the forces has different types of creatures, those that differ in power.
About dying and rebirth.
An obvious pattern says that a creature, destroyed in the reality plane, goes to its natural plane, to later respawn if possible. If it is killed in its home plane, it's permanently dead, dissolving into "matter" of the energy said. That's the case for demons (and Disorder), Old gods (was confirmed in some interview), Titans (they have respawned after their death by Sargeras). For mortals like us: we, creatures of Life, go to Death realm (shadowlands), loa and wild gods (higher level Life creatures) as well - to Ardenweald. Elementals, creatures of 'lesser planes' if we say so, also follow this 'destruction pattern'.
Creature tiers.
My hypothesis says that, for each force, he can point out several tiers of creatures that have different scale of power.
Tier 1 is the strongest creatures, that are the ultimate beings of the Force, with godlike powers, able to create lesser beings at whim. They are pure manifestations of their force and are vulnerable to the opposite one to some extent. Some have shown to have ability to assimilate energies of other forces. Probably that essentially turns them into beings of other forces (Sargeras turned to Disorder, Eonar - to Life, Argus - to Death(?), Azeroth was about to be turned to Shadow).
Tier 2 creatures are usually the creations of the T1, made to serve some exact purpose. There may be its own tiers within this tier, including creations of creations etc. They are made of the pure force by T1 normally, some can be called prime ones (T2.1), like titanic watchers, some are lesser ones (T2.2).
Tier 3 is some mortal races that turned into beings of the said force. The turning is complete when the dying creature goes to the said plane instead of Shadowlands. Note: by tiers I do not automatically mean power level, as sometimes a turned mortal becomes stronger than T2 creature. But normally T2 creatures are stronger than t3.
Finally, the mysterious First Ones, as I suppose, may be the 6 ultimate creatures, T0 level, that surpass even T1, and have complete control over their respective force. They seem to have established rules of the game.
Note that power levels of tiers seem to be slightly different for different elements.
Why can this scheme be useful? Using it, we can chart the missing parts and predict appearance of new beings we've not seen yet.
So let's go.
Arcane
This is nearly the best known force.
T1 - Titans. They have godlike powers and order everything they see.
T2 - all the titan creations. T2.1 are the Keepers like Odyn, Ra etc. T2.2. are: mogu, meachagnomes, dwarves, vrykul, etc. Breakers line, down to orcs, are also part of titanic T2.
T3 - dragons and dragonkin. They are living creatures granted with titanic powers. Races like dwarves, humans, orcs, mogu are not - they are rather T2 Arcane creatures "corrupted" by the Life.
Shadow
This one we know well, too.
T1 - Void lords. Probably at titans level or even above, since they can't even manifest. Though it's not necessarily the reason.
T2 - Old gods are T2.1 creatures of Shadow, and they seem to be stronger than their Arcane counterparts - it took several Keepers to imprison a single Old god. T2.2 are the faceless ones and voidlwalkers. They seem to be of different lines though, voidwalkers are more direct manifestation of Shadow, faceless are creations of Old gods.
T3 - all the manner of void-corrupted creatures like ethereals, k'thir, akir (probably), naga, and lot more.
Death
We're about to learn more of it. From now:
T1 - the Eternal ones aka the Death Panteon. They are said to be a bit weaker than the Titans, though Zovaal (and probably Arbiter) are Titan++ level of power. Still, the Jailer is confirmed to be one of the Eternal ones.
They created their respective covenants and its inhabitants.
T2 - creatures like Stewards, Gargoyles, Fawns, etc. Those that are born from anima and serve some duties in Shadowlands.
T3 - creatures that are made from deal mortals. Undead, first of all, but also Kyrians, Ventyr, Malraxxus warriors. The Lich King is a mixture of both, being made from a mortal, but bears power of T2.1 level creature. Also, the nathrezim are either T2.2 or T3 death creatures, created by Denathrius or Zovaal and tapping into all other 5 powers.
Disorder
Though it seems that we know much about demons, in fact it's almost nothing actually. We see a lot of T3 types, and very few T2. Let's see.
T1 - unknown. Sargeras technically is one, but he's alien to the force. Let's call this a Demon Lord, as to Void Lords. Somehow a titan was able to become a Fel-creature, and managed to bring order to a disorder plane - an organized army of demons. Probably a true Demon Lord in somethins uncontrollable, living in Twisting Nether, extremely chaotic and violent.
T2 - probably the annihilan, pit lords, are one of the few creatures that are pure disorder creatures, born from Twisting Nether, not a corrupted living creature. Also the infernals are, probably imps and felhounds too, but not for sure.
We don't know about the inquisitors, jailers, felguards, sayaad, shivarra. My guess is that inquisitors and ered'ruin vere some Arcane creatures originally.
T3 - all the manner of fel-corrupted mortal creatures, from eredar and fel orcs, to felsworn elves and whatever else. I suspect that felguards and succubi were mortals too, as they look humanoid, like the eredar. Also, dreadlords are the Death creatures from Shadowlands, who willingly turned into demons to play some big game with the forces. Technically, Archimonde and Kil'jaeden were T3 creatures that acted as T2.1 for Sargeras.
All in all, our knowledge of Disorder is really bad, we have huge uncharted territory here, so I expect to see demons in more of the future expansions, to learn of their origins and T1 creatures of cause.
Light
Even less we know about the Light. So few for now:
T1 - unknown, but hinted. Some suspect that Elune is the "Light Lord" type of creature. They probably made the Naaru.
T2.1 - Prime Naaru, like Xe'ra. Seem to lead the way for others. T2.2 - usual naaru.
T3 - all the Lightforged\touched creatures. Prophet Velen, lightforged Draenei, Tyralion probably, Lotraxion (shadowlands agent), Yrel and her crusade. Lightforged orcs, too.
All in all, much more is to be revealed.
Life
Strangely, the least known force, though we are part of it.
T1 - no hint at all yet. If Elune is not from the Light, she must be T1 Life force creature.
T2.1 - one thing that comes to mind is the Overgrowth from Draenor, the supergiants of Life, from whom the Primals come. T2.2 are Wild Gods, loa, lesser Primals.
T3 - mortal races, those who were not titanborn. These are trolls, all kind of stuff that spawned form Wild gods like furbolgs, harpies and so on. Mortals seem to be most flexible as they can assimilate and wield any other force with ease.
I do expect to learn more information about Life creatures in further Shadowlands patches.
Elements.
T1 - unknown, probably none, because these are not the 6 main forces, just some lesser ones.
T2.1 - elemental lords. These are biggest elementals seen yet, they even surpass titanic watchers, as it takes 2-3 to take one down.
T2.2 - regular elementals, with their own power levels.
T3 - ascendants, both from Twilight's hammer and shamans, druids of the Flame, creatures like Alysrazor.
So, what can we expect? We are yet to see all 6 types of T1 creatures, and my bet is that they will be major foes from now on. We'll get to see 'panteons' of Disorder, Light and Life, Void Lords and other types of creatures. That's what we can expect in future expansions, time to cover all the missing parts from this chart.