We seem to actually be saving most of the Pantheon. Also, the current state of the Pantheon is unclear. Sargeras destroyed their world-sized bodies. These current forms are likely to be much smaller, along the lines of Keepers. They'd still be powerful. However, we seem to face only one in true battle, Aggramar. This may not be a battle to the death, however. Considering he is the one that empowered Odyn, who we didn't truly beat, it may be that we're just going to end up pummeling him and surviving him, until he comes back to his senses. Like the bosses in Ulduar. It would make sense.
We may take on a Titan as last boss. The enigmatic Argus the Unmaker. But we're likely to have freed the Titans by then, who are probably boosting us for this encounter. Thus making it a lot more credible.
I do feel all of this is going to end with the death of Argus. Maybe planet-size Sargeras arrives in the final cinematic, and Illidan blows up the planet like a grenade to take him out while we escape. Maybe the death of Argus' world-soul will cause the planet to go unstable. Either way, the planet's death is the easiest way to explain to why it's no longer in the sky, from a storyteller's perspective.
If the picture floating around this thread is of the last boss then that definately can't have anything to do with sargeras. There is no way they are going to have the most iconic villain in wow besides maybe arthas being just a mind controlled dummy. To defeat sargeras we are going to be fighting him somehow directly. He is a demon resembling the devil with fire and horns. He does not resemble a mob from throne of thunder.
Either the boss list is incomplete or we won't be defeating the legion in the 7.3 raid