I Came up with my own personal theories and ideas as to what they would be like because I thought it would be fun (mostly the second legion because reasons).
II- My idea of the second legion is that they were from a water world with the occasional mountain raising out of the depths, on the mountains is where humanity made it’s home after psychicly gifted settlers fled there due to being cast out by the populous, it was inside caves in these mountains that the settlers found luminous crystals ebbing with psychic energy, when harvested and worn in armour or jewellery, they granted a significant boost in psychic potential, especially in the art of fulgurkinesis (shooting lightning out of your body). After centuries these settlers had made a living for themselves by harvesting edible cave fungus and hunting various form such of wildlife in the sea and on the mountains, this was often hindered by the planet’s weather, which consisted of storms Andy tsunamis all year round, leading to these settlers using the winches on their ships and basic spears to catch fish instead of the usual boat transports. They had also cultivated rich ores and rock from the cave systems sprawling throughout the planet’s mountains leading to them attaining forms of technology similar in asthetic to the modern day tau empire (that being lots of luminescence and LAAAASERS... AND LIIIIIGHTNING). all of this would change when during one of the worst storms ever to hit the planet, what looked to be a comet thundered down from the heavens and lodged itself into one of the most populated mountain-cities. During their investigation of this phenomenon, the settlers found that this was no normal comet, but a capsule, that had crashed into a previously unexplored cave filled to the brim with the fulgurkinetic crystals. Upon opening the capsule the settlers were shocked to find an infant child inside, seeing this a their responsibility, a noble of the citie adopted the child, raising him as best he could and teaching the ways of the hunter, the gatherer... and the warrior. The child, whose true name has since been lost in the halls of time, grew to be a Demi-god of a man, standing a monstrous 10ft 1’ and sporting blank white eyes that seemed to glow with psychic energy, he spent his early adulthood defending his home from native xenos monstrosities, whilst also making sure his people were well fed and treated fairly (as he now took the mantle of king). One day, while searching for especially potent crystals, the young man was greeted by a stranger in the cave, a man whose face was hidden by shadow, the stranger asked what he searched for, the young man was wary that he had never known this man, and his immense psychic capabilities alerted him to the fact that he was no native, he raised his family’s most treasured weapon, an ornate golden trident, with a fulgurkinetic jewel embedded in the center, and asked from where the stranger came, the stranger held out his hand, and in it, was a crystal. Perfect in every dimension, it gleamed a neon blue, the stranger said that it was what the man sought, and that he must take it, for it would bring him great fortune. The man cautiously took the crystal and felt its energies surge through him, it felt as if he had been enlightened by some ethereal being, clasping his hand around the gem, he stumbled back, almost attacked by visions of the future, a reuniting for the ages, a brotherhood between a savage and a saviour, and finally, the death of two he did not recognise, but somehow loved dearly. He dropped the crystal, demanding the stranger reveal himself, the stranger removed the hood that cloaked his face, revealing god-like features, a fair yet intimidating visage, and long, flowing black hair. He spoke to the man, who now knew to whom he spoke, as only supposed dreams and nightmares had hinted to who he really was, these were no mere dreams, these were visions of what was to come, the stranger held out his hand again, and said “come with me, for I will show you the galaxy and more, I will show you those who you have sensed, but not seen, I will spread mankind’s glorious light with you and your brothers at my side... son”. After reuniting with his father, the second primarch would be introduced to his legion, his own sons to guide and teach, his own so small who would give their lives for their imperium and their primarch. He would be introduced to the astartes who would soon be known as... the Tide Kings. Over the decades he would fight with his sons to reunify the galaxy, he would witness the rediscovery of his lost siblings. He would confide in the crimson king, for they were the sons with the most psychic potential, he would be there to understand and console the night haunter, as they shared prophetic nightmares, that only seemed to worsen as time went on. But most surprisingly, he would form a friendship with one did not understand nor was understood, a man so beyond reason that even the great wolf of Fenris would seem tame, for when the red angel was returned to his fathers crusade, he needed to be convinced to even fight with his own sons. One had convinced him to fight for the sheer joy of battle, and for the sheer bloodshed it wrought, but there was only one who convinced him to stay with the imperium, to not fight against his own brothers... at least until there was nobody left to stop him from doing so. After a successful cleansing of a xenos world, the second primarch retreated to his private chambers aboard his flagship. It was there he would suffer a vision so vivid, debased and morbid, that it shook his very soul and near drove him mad. He was trapped, he could not move, speak, or even blink, he had to stand and watch... as his own brother struck down the emperor. He awoke hours later, after trying to understand or talk himself into believing otherwise, he came up with a solution, he ask one who may also know of this nightmare... the night haunter. After a long journey without receiving any council from his sons, he hastily boarded the nightfall, and immediately received council with his brother, he approached his sibling without greeting or pleasantry, and asked him if he had seen the vision too... the Night Haunter remained silent, the second screamed in rage, repeating his question as his impatience grew to an exponential level, the Night Haunter slowly turned to face his brother. He said nothing, the second grabbed his brother and demanded that he tell him, he was far beyond reasoning at this point, and any interruption, whether from an astartes, astropath, or remembrancer, most likely would have cost them their life. Curze looked at his brother with sullen eyes... and told him that he had seen the same vision. The second weakened his grasp and stumbled back in pure shock, many emotions came over him at that point... but at the end, there was only pure, unbridled, unstoppable... hatred. He burst out his brother’s chambers demanding that his ship be preparedto return to his flagship. For the second primarhc was now on the warpath. Upon returning to his ship, he ordered his entire fleet to jump to the fleet of the Luna wolves, his plan was to stop the heresy, before it could even begin. His fleet re-entered realspace, opposed to the Luna wolves fleet. The second received a communication from the vengeful spirit... it was Horus, he heard horus’ voice and could not hear his brother asking why he had arrived. No, he heard a traitor who would doom the imperium. His fleet would engage horus’ almost immediately, boarding the vengeful spirit and carving his way through the ranks of the Luna wolves, the second primarch was now beyond hatred, he had now started hallucinating creatures that could only be described as daemons, creatures wearing the armour of the Luna wolves, but had sprouted horns as black as night, fangs as sharp as razors, and claws pointier than a master crafted sword. These creatures were animals, and they had to be put down. He tore down the doors to the command deck, and there horus stood, alone, waiting. The second had revered Horus, and thought him to be a good man, a man of strong will, brilliant resolve and undeniable charms. Now however, he saw betrayal in human form, and it sickened him, horus spoke, but his brother would not listen. Their weapons clashed, throwing sparks across the deck as they were beaten together violently and savagely, horus was not ready to kill his own kin, and thus stayed on the retreat most of the duel, this was ultimately his downfall, as the second disarmed him without remorse or mercy and knocked him to the ground. Horus did not want to fight his brother, he was not ready for such an unexpected betrayal, he felt nothing but sorrow in his heart for the one he called “friend” now stood before him, ready to kill. The second raised his trident as if to kill horus. But he looked at him, he took one, single look at him. He realised, how could he do this, this was his brother, his own flesh and blood, his friend whom he had seen the most frightful horrors and endured the most trying times with. This was his family, not his enemy. He dropped to on earth knee and wept, only a few short minutes ago he had seen nothing but red, hatred had clouded his mind, word had already reached the nearby fleets of the imperial fists and them emperor’s children, who had just arrived in response to the assault, he gazed to the raging battle outside and knew, there was no redeeming himself, he had single handedly destroyed him legion, his reputation, and his own psyche. Two squads of tactical marines burst into the room, upon seeing the downed Horus, the immediately opened fire. And for one last time, the second primarch looked to himself brother in horror and whispered... “I’m sorry”, right before a Bolter shell pierced not only the chest of the primarch, but the gem embedded in his trident. The gem exploded in a storm of psychic fire, the second son howled in pain as he was dragged into rift in time and space, never to be seen by mortal eyes, for millennia to come.
That is some theory of yours but it might be an accurate one too. I don't know about the astartes that they would be the primarch over but that is to be seen if they make an appearance. How do you think if the missing primarchs would make an entrance into the current state of the Imperium and the universe, and the effects they would have?