Yes, it turns out that Xavius, the Nightmare Lord and first Satyr, who has always been affiliated with the Emerald Nightmare, is not going to be the raid's final boss after all. In fact, he'll essentially be the boss of the first raid finder wing.
This could have serious implications.
As a satyr, which in WoW is a species of demon, Xavius ought to be affiliated with the Burning Legion. It was he who actually made contact with Sargeras before he notified Azshara, and he wound up being the first Night Elf to fully pledge himself to the Legion (not sure when Illidan did, but I'd argue he didn't "fully pledge" himself given that he stabbed them in the back.) If Azeroth had gone the way of Argus, it might only be known as that world where Satyrs come from.
The Emerald Nightmare itself has always been thought to be the work of the Old Gods. I've theorized that N'zoth, the big no-show in Cataclysm after we stomped C'thun and Yogg-Saron, and we'd later find out Y'shaarj had been (only mostly) dealt with eons ago, is actually in the Emerald Dream. Essentially, if the Dream serves as Azeroth's back-up copy should Algalon decide to trigger the Re-Origination device, N'zoth's well situated to just pop into Azeroth 2.0 (we assume only 2.0) and be king of crazy mountain.
How did a demon wind up so associated with the Nightmare if it's an Old God thing? We know that these two do not play well together. (In fact, I almost wonder if there's a rock-paper-scissors thing with Demons, Old Gods and Titans.)
Perhaps Xavius' job was to turn the Nightmare, to purge N'zoth but replace the Old God with his own demonic essence. The Nightmare perverts the Dream, to be sure, but the Legion probably wants any sort of back-up copy of Azeroth destroyed with the original.
So who could be the final boss of Emerald Nightmare?
I feel like possibility number one is N'zoth.
We've gone for a long time without fighting an Old God. In a way, that makes sense, given how powerful they are. On the other hand, it seems as if we've never truly killed one (though I reserve judgment on C'thun, given that eyeball-covered Cho'gall looked a lot like eyeball-covered Garrosh, and beating Gary eliminated the last remnant of Y'shaarj.)
I really don't know anyone else who could be a satisfying final boss for the Nightmare if Xavius is off the table.
Really, I think we need to know what the Old Gods are going to be up to with this Legion invasion going on. Their lives are on the line just as much as ours are. We're supposed to find the only way to defeat the Legion in 7.1's Karazhan content. I wonder if we might have to fight lawful evil with chaotic evil.
Essentially, perhaps our foray into the Nightmare winds up making things worse, rather than better.
Oh who knows. I'm just really excited about this expansion, guys. Like, not since Wrath have I been this excited.