Good ideas btw
You may recall that John Stone is also a Necromancer with a specific legendary amulet. It's common knowledge that evil is perceived as the dualistic antagonistic opposite of good, in which good should triumph and evil should be defeated. The symbol of the twisted snakes or Caduceus is symbolic of this duality of light and dark, good and evil, life and death, and health and poison often misapplied to medical symbolism, which is actually in the symbol of the Ascollapius.
These symbolisms, or what have you, binds this to the Necromancer, and is a very obvious aspect of this character. Even Ningazida's aspect of herald to the earth mother, puts him in very close relation to Lilith who is called creator of sanctuary or even the first mother. However, that would have all been well and good and a perfect defense of Magnus' identity as Rathma, had they not changed these key aspects of imagery.
The alterations were subtle, yet significant enough to almost certainly confirm this is not Lilith's son. And although he calls her blessed mother, an era of reverence that Rathma doesn't have for Lilith doesn't actually mean she's his mother, and quite possibly, an adopted parent, something like an orphan. This would be more in line for the interesting choice of the name Magnus.
Many commenters have attempted to state that this is in reference to his obvious role as a mage, saying that this is a term meaning, spellcaster. That term would actually be mages, and while Magnus, loosely means 'great' in Latin, is why that name refers to the tall man. In diablo's history we have we heard the name Magnus before. The answer lies in a game, which is very central to the inspiration taken by the development team of diablo iv. In diablo 2's expansion, Lord of Destruction, we were introduced to a multitude of concepts, which could come to pass in diablo 3 and diablo immortal. Most prominently the Nephilim and the world stone.
Additionally, we were gifted with several new armor and weapon sets. Most importantly, the orphan's call set of armor. Within this set, is a pair of leather gloves called Magnus skin. But, no other information is offered beyond that, leaving us to find connections, among the circumstances surrounding its appearance. Orphan? Could this be a reference to the purge, when Lilith went on a rampage to destroy the parents of the Nephilim who had plotted to destroy their own children to avoid the wrath of the high heavens and burning hells?
Who, most likely would have sensed their offspring's power and come to sanctuary? Now it certainly wouldn't be a far stretch of one's imagination to say yes, and would certainly account for Magnus's reverence for Lilith, in his belief that she could save them, or that he needed her help. Maybe. Whoever, them is, while this does link the concepts together, there is very little information to grasp onto with reference to the history of Magnus.
It could very well merely be a new character that needs to be fleshed out, which can only be predicted after the game is released, to see if this is in fact the perfect name, Magnus, as chosen. Other facets of this character may yet belie some level of information and revolve around small details surrounding his design. If only, we could put a finger on it wait a second, the tall man's finger, along with references provided by current and past diablo games, one item in particular, brings a very intriguing set of disclosure. A witch doctor's specific ring, the tall man's finger tells the story saying this finger was severed from the hand of an Umbaru priest who crossed over to Mbwiru Eikura in search of knowledge and came back and changed. Before the unclean phantasm was finally slain, he evinced great power over the dead and this metal-plated digit still possesses some of that power.
Seen in the d4 trailer of the tall man's left pinky finger, which seems to be a particular focal point for viewers, as the camera zooms in on the joining of his hand with Lilith's, clearly showing the missing finger capped in a metal adornment. The director chose to focus on the left hand intentionally displaying this missing digit!
Now, alone this would be a very weak argument but the references behind the tall man leaves us with yet another set of intriguing insights. Again, the development team has done an incredible job of sneaking in references and inspirations. The ring's reference is clearly related to the character to the tall man of the phantasm series in movies. The unclaimed phantasm, is a good start.
As far as appearance, there's no real parallel, except for the penchant for black attire. The tall man's history is also directly linked to the item's description with a few nuances. He was a mortician who travels back in time after becoming fascinated with the connection between the living world and the dead. The portal, subsequently returned him to his time, and he changed. With this change, he now possessed superhuman strength, telekinesis, and the ability to shapeshift. And, he also commanded power over the undead, and even demonic figures.
Now that sounds pretty familiar, doesn't it? Even the references to the metal plating has to do with the tall man's use of metal spheres, and the fact that his severed limbs can become something else altogether, while still containing his power.
Clearly, they're seeking to build a new character, instead of including an older one, like Rathma. One other appearance change, that gives us insight to the knowledge of Magnus's identity. Although it is a subtle change, it is the strongest reference point that can be gathered. It is telling to see the change to Magnus' identity, by the swap of his amulet from the caduceus to a pentagram. These references are cited in cultists video on the channel. This is canonically the symbol associated with Andariel, the Maiden of Anguish, as pointed out in another comment section I read, that the cut quest for Andariel in diablo 1, saw the drop of the maiden's brooch, which incidentally has an icon of a pentagram. I guess, a bit of nostalgia recollects old memories in a rosy, positive light.