To me Sanguinius is dead, but death of such a creature as a primarch may have different rules.
I could see supernatural aspect of a primarch work to bring them back.
Perhaps the primarch as a warp entity that shares some of it favour through the gene-seed.
Kinda like a chaos god and mark.
So each act a space marine does in the name of his primarch boosts the "spirit" of the primarch.
And give him strength in the warp.
But that in itself will do nothing, it just surplus power that is waiting to be used.
Waiting for the right catalyst to break the natural order of things.
Look at Guilliman, all his "sons" fighting in this name.
If that would give extra power to define the natural way of thing, then all he would need was someone to flip the switch and allow him to return.
Same could be done with Sanguinius.
All that fighting and sacrifice could be empowering the spirit of Sanguinius.
If he has enough power then someone could summon the spirit back his body and allow it to restore it and bring him back from the dead.
Think of it like summoning as banish demon, it has to be back in power and you need the right amount of sacrifice and items to bring summon it.