There's are a couple of rumors floating around from Games Workshop about new Warhammer 40k rules and models releases; as well as major changes to the fluff.
Rumor 1: All of the Primarchs will return, including the dead.
Rumor 2: One of the traitor Primarchs will become a loyalist and one of the Imperial loyalists will become a traitor.
I got into a discussion with some people on a thread on Spiky Bits' Facebook page about it and here are my theories.
The Emperor will briefly return, only to be killed again. Once he finally dies, all of the pysker souls he's been consuming on the golden throne will coalesce and bring all of the dead primarchs back to life. The missing primarachs will rush back to Holy Terra like moths to a flame. Of course, this means both Horus and Sanguinius have returned as well.
Horus will see the results of his actions and will repent his wicked ways, blame Ebrus/Logar and the Word Bearers for tricking him into corruption and turn to the side of good. He then will wage a new holy war against the other traitors and will probably kill the Chaos god, Slaanesh.
The return to life will not go well for Sanguinius though. The shock of being betrayed by his closest friend, returning to life, and seeing his father die will force him to succumb to the black rage and the Angel will fall to the side of the traitors. The psychic link to his "children" will send all of the Blood Angels into the black rage and they will turn on the Imperium.
That's just my thought.
Most people seem to believe that the Alpha Legion will be the traitors to turn good because they were lead to believe that becoming traitor will help humanity. Some folk also give good odds on Magnus turning to the side of the Imperium because he was basically duped into becoming evil and he was trying to help the whole time.
The Space Wolves seem to lead most theories that they will make a heel-turn and become bad guys. They are one of the more aberrant of the loyalist forces and are already treated with suspicion by their brother legions but I think that's a bit obvious.
I really think the End of Days for the 40k universe will come full circle and end with Horus and Sanguinius.
What do you guys think?