The first and biggest question surrounding his return is HOW? Well, turns out it would pretty much be the same as Guilliman.
See Sanguinius is in a sarcophagus in the Sepulchre of Heroes (thus why I am extra sensitive to this). Now Blood Angels sarcophagi are NOT what they are today, full of mummies and dead bodies in Egypt. These are slightly more akin to the tombs that the Vampires live in in the Underworld series, just not in the floor. They keep the occupants alive and can restore them and are similar to a dreadnought sarcophagus. Not only this, but they are also what Neophytes go into for a year to transform into battle brothers (which means there are likely BA recruits who were on Baal and missed its whole devastation. Funny extra thought). And Sanguinius' is the EXTRA SPECIAL Golden Sarcophagus, so who knows what that one can do.
But basically, it would boil down to "Sanguinius is in stasis, healing, in his Sarcohphogus. Lets open it up!" and then once they do, it would go something like this:
And lets just say for now he is, this opens a few cool doors
1- The visions Death Company get are actually psychic projections of the trapped primarch going mad.
2- If he comes out, he will be insane and we will have this Death Company-esque Lord of War choice for the game.
3- We'd likely get an answer to exactly WHAT the current Sanguinor is.
4- Probably give Cawl another chance to be AMAZING by bringing Sanguinius back to "Life" and making some really new spiffy amazing model for hawk boy.
"Cool, case closed" you may be saying. Well hold your horses there partner. This is where we start ripping open plot hole big enough to drive a land raider through.
So here is a list of all the holes opened with Sanguinius being in stasis and not DEAD.
1- Is Rogal Dorn really that stupid? Or is he just a massive MASSIVE dick.
So the story goes Sanguinius, the Emperor and Rogal Dorn all teleported onto Horus' flagship, the Vengeful Spirit, during the end stage of the Siege of Terra. Once onboard Sanguinius found Horus, fought him and was killed. Then Big E found them both, fought Horus, killed Horus and was mortally wounded in turn. Finally Rogal Dorn walked in, said "Oh crap," and then took Big E and Sanguinius off the ship.
Now Rogal KNEW the Emperor was still alive. And he KNEW Sanguinius was dead. Not in stasis. Not dying. D. E. A. D.
A Primarch should definitely be able to identify these things through their preternatural abilities (including PSYCHIC POWERS) and just general "I'm better than you at everything"-ness.
SO if Sanguinius WAS alive, Rogal should have, beyond a shadow of a doubt, been able to tell. HE WOULD HAVE KNOWN. Which means if he said nothing and just dumped the body off with the 9th Legion, then he is at best the galaxy's biggest jerk and at worst, a self centered traitor eliminating Sanguinius as competition.
2- Is every BA Librarian stupid, blind or inept? Psychic powers people. From the moment he fell all the way until M41, how would no one have psychically sensed that Sanguinius was alive? Mephiston is one of the greatest psykers of the 41st millennium. He should at least be able to tell. But in 10,000 years, NO ONE has said "Hey... wait a MINUTE," and figurered out that Sanguinius was in stasis and not dead and that the visions the DC experience or the noises the Sanguinary Guard hear when guarding his tomb ARE ACTUAL REAL PSYCHIC PROJECTIONS OF SANGUINIUS.
3- His psychic death scream. Supposedly this is the catalyst for the flaw. Every son was triggered the moment he died, and now that curse lingers in their blood FOREVER.
But the key thing here is it was a DEATH scream that did this. Not a "Mortally Wounded" scream or "Going into coma" scream. No. It was a DEATH SCREAM. Without it, what causes the flaw? I think the Black Rage/DC fluff would need to be reworked, and that really isn't an option is it....
More important than the plot holes though are the implications.
From a game persepective, we have potential Primarch Parity right now.
Horus and Curze are dead as can be.
So are Ferrus Manus and Sanguinius.
Now there is no way the first 3 are coming back. They for sure are dead, with no bodies left either. So that leaves 2 Traitor and 2 Loyalist gone and dead. Eventually, if they all get released, you would have seven loyalists and seven traitors. If you bring back one on one side, you HAVE to do it for the other for the sake of balanced numbers. And that would lead to some pretty awkward discussion surrounding the FOR SURE dead primarchs.
If Sanguinius lived, it would invalidate his visions of his own death against Horus,
It would largely invalidate the nobility of his Sacrifice. He is such a great hero because HE KNEW he was going to die, and faced Horus all the same. This sacrifice is a driving theme of the Blood Angels. This doomed nobility. To retcon the death would be to diminish the character of the legion.
It would also, diminish the current Heroes IMO. I've talked about this before, so I won't rehash too much, but really it comes down to lessening the achievements of mortals because gods now walk amongst them. What can they achieve when a Primarch can do so much more?
These last two points above all else, despite how amazing the model would be, are why I want to let the dead lie and leave Sanguinius in his tomb.