Hmmm, I dont know about another Horus Heresy event, I dont think the Imperium can survive another galactic-wide rebellion. With Guilliman in command, the Imperium can have the direction, to finally be on the offensive instead of the defensive. We must also remember the traitor Primachs will also have their own selfish desires, and is doubtful that they will be cooperative. Where Angron is angry at everything, Mortarion straight up hates everybody despite his patron god's more jollier disposition.
As for the returning Primarch I think Vulcan will come out far later, as the Salamander's are still looking for his relics. In fact I think the returning loyalist will be the ones that we least expect; Jagathai Khan returning from the Webway after fucking the Dark Eldar 90 ways to Sunday, and Corvus Corax when he finally stops being emo enough and straight up help someone, he ad pretty much 10,000 years to brood and wallow in self guilt. That damn asshole Lion will probably be one of the earliest ones to return before Leman Russ, what with Cypher finally on Terra, the mystery surrounding the sword is one worth paying close attention.
While this also is the most unlikely...mayhaps can...even Sanguinius rise from his tomb...I can only hope they do, even if its wishfull thinking.