I have a farfetched ridiculous epic theory...
I have an epic theory I'd like to share with everyone
My theory is, how cool would it be if the ALpha Legion were planned to be one of the 'Big Bads' of an edition in the future?! Now hear me out...
When Alpharius died Omegon felt his death, and apparently Omegon died at the hand of Guilliman, I actually quite like the idea that both are dead but they had seen a future of saving mankind by destroying the Imperium however they both needed to die. When they both died it would send a complex psychic/warpy vision, like a genetic flaw in the form of an activation phrase that simultaneously subconsciously enters all AL minds. However, prior to their death, Alpharius and Omegon hypnotised and sent 100'000 astartes into other factions to infiltrate. THEN when the activation phrase is broadcast across the galaxy it will wake up all these sleeper agents in every corner of the galaxy: in SM factions, chaos factions, multiple planets and star systems. Hence starting the mass back-stabbing of the 12th Edition Big Bad Alpha Legion.
At some point, Akurra in the recent books finds out what this activation phrase is, but when he hears it he understands it isn't the right time to use it - but he is shown/sees that point in the future.
When that time comes he'll broadcast the activation phrase across the galaxy, maybe via the warp somehow? (It could be some badass line like The Heads of the First Hydra Breathes Again)
And the best part? When Alpharius and Omegon died their consciousness/soul was trapped in the warp, waiting to hear this activation phrase. And when it does, it reawakens and enters the mind/body of a legionnaire turning him into a kind of warped Daemon Prince, or maybe even just a juiced up Custodes sized Astartes. Then when that one dies it transfers to another legionnaire... Kind of like a watered down Avatar of Kaine situation - or like the myth of a Hydra's regrowing heads.
Tell me what you think, Brothers