I see three options for this to occur.
1. The Thrist:
When he was alive he was the embodiment of what Man could strive to be. However the killing blow essentially did just that it killed him, but here in lies the complications of death for these things known as primarchs.
They are more, more than man. more than what man could be. So the laws of man might not apply.
All the sons of Sanguin have the thirst and are based off of vampire lore. In some lore vampires can be near death and appear dead, but when drenched, and feed a constant supply of blood they can rejuvinate things that would kill anything else. Additionally some lore has vampires skipping through time by being drained of blood and getting re awakened by blood.
If Saguin comes back from this then I expect him to be a dark version of himself, with a thirst for conquest and blood (literally).
--- I personally wouldnt like him coming back this way.
2. The Greater Emperyean
His Mortal self is dead, the flesh gone and whithered. However his soul is something more and has slowly awakened with the emperor in the warp.
There are things in the warp that fight chaos, Angels made of fire. What if these are shards of sanguin, or his minions.
He has become essentially a greater deamon of the Emperor, he now appears across the galaxy in multiple fields at once, and acts to save his kin.
-- I dont really like this option either.
3. Mephiston, the Avatar of Saguin
Mephiston is changing, into something more has been rumored in the lore for a while. What if the final step for mephiston is to feast upon his primarchs flesh to undergo the final step.
Mephiston gains some memories of Sanguin, he also changes physically, growing wings, growing, physically growing to primarch size.
Mephiston is no more, yet he is not Sanguin, but something inbetween.
- I like this the best
4. Sanguinor is sanguins spirit made manifest, and he soon will undergo another transformation and become a greater empyeran