I hope i'm not making a thread in the wrong place, but i see this more as a possibility, and lot, LOT, of speculation.
Starting with the vacuum of Nicol Bolas after the endgame of War of the Spark, we had some space to breath with Zendikar Rising, but then comes Kaldheim, properly speaking, Vorinclex, the sub-plot. Not very subtle, but brews one new "epic" to come (as a story, not a moment of feat itself).
Strixhaven put some cold water, to give time for this plot to grow (hope we have a proper good allied colored set that differs what Ravnica defined). Innistrad came to life (hah), then Kamigawa was correctly managed to flow with the rest of the sets. Oh, yeah, Jin-Gixatias's plot, now in the making of being the main plot after all.
Just in the next, New Capenna, Urabrask, the one stranger who actually have common sense between the "brothers".
And again, the next, Dominaria greets Sheoldred, the leader of the barelly mentioned Seven Steel Thanes.
Brothers' War, the retcon, is the last calm before the storm. After all, All Will Be One is the next with the last Praetor, (Enuma) Elesh Norn.
Everything goes down on the March of the Machine, and we have glimses of new and some historically-known-but-not-seem-before planes, as well as some forgotten for a while (love ya, Lorwyn).
Now, my point, finally.
Vuel, the one that will be the Volrath, after losing his pride, eventually goes to Rath, a plane made by Yawgmoth to invade Dominaria. Became evincar, changed name, start revenge, was betrayed, cheated, killed, and almost entirelly erased from history. What a boomer.
In the middle of his messy life, he did (as i suppose through a temporary overlap of Rath) find a mysterious plane where slivers live. He, as a nerd, take the Queen to Rath as a extra layer of security to guard the Legacy gadgets, and some beings to study, creating eventually the first artifact one, the Mellatic Sliver. Seems that he had boosted their ability to change, and use his creations to control from within.
Now, Karn convincing the Queen to me was a plot armor. Every one being connected via hivemind doesn't change that they are alive, and Karn compared them with his doomsday machinery, but he will use for good. Yay. I think the Queen allowed because she disliked Volrath in general. But anyway, she died with others when the Overlay happened, if by volcano or by the wars, she's gone. Riptide nerds, hundred years latter, find fossils, remake them, create new types, but with no Queen, chaos ensured. Mirari, with its deus ex machina powers, created Brainiac Lord with some survivors of Karona's destruction.
Some survived by time-shifting, and became ronins, losts without a proper guide, consuming everything possible and growing in numbers. they become a nuissance, but not to the point of losing control. Some tried to reign over them, but was short lived, and Hivestone was just a reverse card at this point.
They flourished even in the "what if" futures, and the Legion proved they might not need leadership any more.
Now... the mess of Shandalar... the Prime ones, humanoid, plated, and seems to have "plates that glisten and slide about like oiled pieces of machinery". They made the revolution to care only to your own side. Logically is better, but looking to the Lore (be quiet, Matpat), this might be the result of some freedom and self-interests. But because they have the Hivelord, i might argue the titan is the reproductor, not the facto-leader. So, with the treatment of the "old shaped ones" be called less brood, i believe the Prime ones have a high level of intellect and individuality.
The First might be the original source of all, but because of the others showing large connection with Alara's mechanics, might be the original creator on this plane.
Now... to end... Tempered Sliver, a Slith-like, which is a Mirrodin being.
Unknown plane, goes to Rath, Dominaria, one day Shandalar and maybe Alara and Mirrodin by back tracking with Karn somehow. Whatever the case, the Queen were lost, and we doesn't know the use of the Lord made by Mirari may had, but clearly coundn't be useless, even less, stupid. Not only 4 visible heads, they rules with purpose, almost as if Mirari granted they most pure, natural desire of leadership, and was done. During the chaos, and maybe notion of self-awareness (after all, the Queen communicated with Karn), the Lord might had seen the problem of complete dependence to a Head figure. With the death of Karona, rifts left and right caused by all crazy stuff since the Brothers' War to this day, and Teferi not helps with time manipulation in Tolaria, at one point the Lord might have seem the multitude of planes and futures through the cracks, and guided his brethren to the start of actual freedom, culminating in the unionization of the Legion, and maybe slipping to Mirrodin with Karn when he's gone to make Memnarch, and other planes.
If i take the props of Shandalar, they might be the logical conclusion. With the power of infinite possibilities physically speaking, if they developt intellect without a mastermind, but maintaining the hivemind, they will naturally have the same potential, but mentally... like humans and human-likes. If they can evolve to be actual freaking magma, ghosts, living mists, and incorporate other life form like fungus, why not the Primes cound be immune to glistering oil, using the benefits of being half-metal, like the five human tribes of Mirrodin/Phyrexia?
The March of the Machine is, in the end, the perfect moment to Slivers move and go to every single Plane possible, unrestricted, and to grow endlesly. Might also gnaw on the Realmbreaker at one point, or do something that pierces planes and maintain a hivemind within blind eternities.
I don't know. I just see they as a "villain" with the most annoying difficulty to end, because all the heroes need to erase the species to its last member, in all planes, and each and every artifact and lab research altogether. Nearly impossible.