The Old Gods and Void Lords do deal in half-truths and obscured prophecy. It might be something like that.
It -is- true that he got cut, but what if he didn't get entirely cut and just -shifted-? Rather than finishing out the Wrathion content in Legion and launching it, they could have Wrathion's part of the war be told through flashback quests where he talks about what he did and the player takes control of the Wrathion in the flashback to act-out his various actions. Like in the Jade Forest with the Jinyuu and Hozen?
That way they could carry forward all the work they did on Wrathion, keep him a part of the story, but shift it forward to offset the workload that was Legion.
Ooooo... I like that a -lot-. And he -has- been getting more active in the world. Plus Kel'thuzad is still drifting about and the Cult of the Damned is stirring up trouble...
What if it's N'zoth this expansion and Bolvar in the next? I suggested a couple of months ago that they could intro Bolvar and the Scourge as an ally to the fight against N'zoth and then have him go bad once N'zoth is downed, with the Scourge already out of Northrend he could just go directly from kicking N'zoth's ass to kicking ours in preparation for 9.0's "Return of the Lich King"?
"The Boy-King sits at the Master's Table. Three Lies he will offer you."
The third death thing is, clearly, Alleria. And -probably- won't happen until some time in the next expansion or the one after it, I suspect.