As you know, Dragonflight has a lot of elemental themes. Therefore, i came up with an expansion concept that would have us visit their realms - not the prisons made by the Titans for them, but their original elemental planes.
The elementals invade the different cosmological forces in search of souls. Their lack of it means that when they die, they die for good. There is no elemental afterlife. By imbuing themselves with souls, they ensure their regeneration after death. But, by claiming the souls of others they also deprive them of an afterlife. That means that when you die, you don't go to the Shadowlands, a demon doesn't go to the Twisting Nether, etc, etc... therefore, we must stop them from succeeding. Otherwise, the balance of the different cosmological forces will be thrown into a turmoil.
Realm of Earth
The pygmies of the Realm of Earth invade the Homeworld of the Titans. As you know, Titans possess a soul, as was the case of Norgannon weaving a spell to protect the Pantheon's souls from Sargeras' Felstorm. Although they are not as strong as a Titan, the Pygmies can manage to overwhelm an individual with their great numbers. Like the story of Gulliver, they tie down a Titan ,which is roughly the size of a Titanic Watcher, and carry it to their Realm of Earth to be stored in the Vault of Souls.
The Pygmies of the Realm of Earth also wage war against the Cranes of the Air Realm. Being of opposing elements, the Pygmies and the Cranes have been at war for millenia. While the Pygmies ride goats and use spears and slingshots, the Cranes launch swift aerial attacks. Based on your Realm allegiance, you'd either help the Cranes or the Pygmies.
The Gnomes of the Realm of Earth guard the underground Vault of Souls. This is where they store the souls they steal and will serve as a dungeon where you and titanic allies will try to get them back. Through this dungeon, you will learn that Gnomes are originally Elementals of Earth. When the Titans encountered them and their marvelous feats of technology, they decided to create their own version of them - ones that are made of metal. Like the way Odyn created his Val'kyr based on the Kyrian, this is how the Mechagnomes were conceived.
The Hobgoblins of the Realm of Earth serve as the guards, stable masters and craftsmen of the plane. They are not the dumb oafs of Goblin society. Mischevious pranksters, they sometimes like to play jokes on the denizens of the Realm. Their ability to shapeshift means that they often get away with many misdeeds. We, therefore, need to catch on the act those who stray away from their duties and bring them back in line.
Realm of Water
The Nymphs of the Realm of Water invade the Emerald Dream. Specifically, the section that is most associated with wet dreams. Their victims are mostly young Druids who venture into the Emerald Dream to learn Druidism. Using their enchanting beauties, they lure these inexperienced Druids into their Realm of Water to either harvest their souls or copulate with them to create an offspring that possesses a soul.
The Undines of the Realm of Water use a similar method, albeit slightly different. They invade the section of the Emerald Dream that is mostly associated with adventurous dreams. There, they lure sailors and seafearers to their deaths using their enchating voices. Entranced by their singing, these adventurers steer their ships into reefs or cliff rocks. The crash allows the Undines to either harvest their souls or copulate with the survivors in order to create an offspring that possesses a soul.
The Homunculus of the Realm of Water are the offsprings produced by the Nymphs and Undines copulating with a mortal who possesses a soul. They assimilate into mortal affairs and politics and are hard to detect by normal means due to the soul that they possess. We must expose them and remove them before they influence our societies for the benefit of the Elementals.
The Dryads of the Realm of Water harvest the dream stuff from the Fauna and Flora of the Emerald Dream, much like the Vorkai of Ardenweald. Similar to anima, the dreams that nourish the environment of the Emerald Dream constitute part of a soul. With enough dream essence, they can form a soul for themselves.
Realm of Air
The Sylphs of the Realm of Air invade the Shadowlands. They are remarkably human-like and, therefore, are able to easily pass off as human souls. There, they abduct the fresh souls that arrive to the afterlife and are oblivious to the whole situation and carry them to their Realm of Air using a pair of wings they sprout from their backs.
The Satyrs of the Realm of Air use a different method to achieve their souls. They appear really friendly, holding parties and festivities to anyone they encounter. Using music, dancing and drinks, they wait until their guests are completely drunk before shoving them into a sack and kidnapping them into their Realm of Air. Some Satyrs, however, found an easier and effortless way to get the souls they covet. Able to resist the enchanting beauties and voices of the Nymphs and Undines, they invade the Realm of Water and forcefully take their souls, leaving them empty handed.
The Fairies of the Realm of Air bait unsuspecting souls into their demise. They infest lonely roads and empty woods, hoping to entrap an unlucky soul that comes across them. They use their charm to draw the attention of any passersby. Appearing like joyful little spirits, they skip around laughing or humming, which piques the interest of whoever they catch the eyes of - especially young ones. But, their cute faces and friendly demeanors hide a nasty set of sharp teeth, which they use to devour their prey. Those who are led astray by them meet a quick and gruesome end.
The Cranes of the Realm of Air are the military force of the plane. They are sent to the other Elemental Realms to establish domination - especially the Realm of Earth, which puts up the most resistance. They also guard the Realm from any invaders and are stationed as the gatekeepers at the entrance to the domain.
Realm of Fire
The Salamanders of the Realm of Fire invade Argus, the Legion's main stronghold in the Twisting Nether. Being able to extinguish fire with their bodies, they pose a threat to any demonic world they arrive at. Without felfire, these worlds weaken the demonic forces of the Twisting Nether. To spare their strongholds from the consuming Salamanders, they have to sacrifice some of their demonic souls for the Elementals of Fire.
The Vulcans of the Realm of Fire are the blacksmiths of the plane. They create the weaponry and armor for the denizens of the realm. To protect their fellow elementals from the foul magic of their enemies, they create specialized armor found nowhere else. To give them an advantage in battle, they forge weapons of elemental fire that burns victims to their very core.
The Genies of the Realm of Fire get their souls in a similar manner to the Demons of the Twisting Nether. Being fully aware of their ambitious and power-craving natures, the Genies offer them a deal. Allow the Genie to possess their bodies and be granted a wish. Many demons choose to accept the deal for a chance to get power.
The Familiars of the Realm of Fire serve as the Realm's assistants. They are in charge of the various manual labours that need to be done around the Realm of Fire. The Genies often send them to collect the ingredients needed for their spells, the Vulcans use them as tool carriers and they are entrusted with keeping the Salamanders' skin clean and potent.
Realm of Spirit
The Realm of Spirit is housed by plant creatures. They invade the plane of Light; more specifically, Karkora, the homeworld of the Naaru. Due to the plane's strong presence of light, the elementals of Spirit grow in power and strength.
The Ghillies of the Realm of Spirit employ guerrilla tactics to get the souls of their victims. Their plant-like appearance means that they can blend into the environment and camouflage themselves within a dense foliage. When a victim approaches, such as a member of the Army of the Light, these plant creatures seize and pull them into the undergrowth where they harvest their souls.
The Hederas of the Realm of Spirit have the ability to shoot vegetation out of their mouths. They use this ability to latch onto a victim and suck the soul out of their physical bodies. Once emptied, the soulless husk is then raised by the Hedera as a plant creature in the service of the Spirit elementals.
The Mandrakes of the Realm of Spirit are nasty little creatures. They burrow into the ground and infest a zone with their numbers. Those who try to get rid of them soon find out why they are so reviled. Once pulled from the ground, they emit a high pitch scream that kills anyone within range. Those that survive are driven into a madness that usually drives them to give up their souls. The scattered bodies are then harvested by the mandrakes for their souls.
The Spriggan of the Realm of Spirit are used as the bodyguards of the plane. Possessing the strength of giants, their small statures often confuse many who face them. Able to grow into tremendous sizes, they are a force to be reckoned with.
Realm of Decay
The Wendigo of the Realm of Decay invade the Void planet, K'aresh. Although Void creatures do not have souls themselves, they do keep them in their possession. These Wendigos are not the ones of Dun Morogh or Northrend. They resemble the Wicker Constructs of Drust society. Having the appearance of an upright, decomposing and disfigured deers, these Wendigos invoke feelings of insatiable greed and hunger, the desire to cannibalize as well as the propensity to commit murder in those that fall under their influence. It will be revealed that they are the ones who afflicted Galakrond with a ravenous appetite. Using their influence, they drive Void creatures to consume themselves or one another, leaving the souls in their possesion for the Wendigos to collect.
The Grim of the Realm of Decay spread diseases wherever they go. Having the appearance of mangy wolves, their postures are distorted, their bodies are enlarged and they lack patches of fur and a tail. Bringing death with them, they terminate whatever they come across, leaving the Void with nothing to feed on. Starving the creatures to death, they collect the souls left behind by them.
The Dhamphyr of the Realm of Decay gorge themselves on whatever they can consume. Bloated and dark skinned in appearance, blood runs down their nostrils and mouths and their nails are crooked and elongated. They prey on the creatures of the Void, absorbing them into their bodies much like Gorefiend in Hellfire Citadel. With enough luck, one of the creatures they consume will have a soul in its possession.