As much as the heretic in me is beggin me not to do so, I disagree. If properly handled, Russ might be one of the most interesting Primarchs to return. No longer the Russ that left, but a wiser and somber Wolf King, battered by his time in the Warp, but still the fierce warrior. A Russ that no longer wants to kill Magnus for the Imperium or for revenge, but because he understands now that both him and the Crimson King were manipulated, and feels that he needs to end the creature that was once his brother, because he feels partially responsible for his fate. An Odin-like Russ, the Wanderer, who's faced his demons and returns ready to play his part in Ragnarok.
Hope it’s not Russ; there are more interesting Primarchs to return than the dog-in-chief.
As for traitors, Fulgrim has been rumored for a while now.
There used to be old fluff about Russ’ wargear being found, even his armor. At a temple of chaos in the eye no less. Not sure if that has been retconned. If not, the return of Russ might be an interesting one indeed, but maybe not in the way most are expecting.