Right now people just feel like it's him and him only. During Mists they said they wanted to go an expansion without saying who the end boss would be, but had to go back on it because of how Pandaria felt and with no "real threat" at the time. I wouldn't be surprised if they are attempting to do it now. They built the expansion up and around the Legion. They've never hinted that Sargeras would be the final guy and left it up to us to assume. We all know from in-game quests (even the tomb ones) and such that we will have to fight Sargeras at some point to "end" the burning legion. The question is, are we officially taking the fight to him or are we going there in hopes to fight him and find out there's been another puppet master under Sargeras control that has been overseeing this whole thing in the event Kil'jaeden swapped sides or died. We won't know until the fight actually occurs.
That doesn't say anything at all though other than an avatar, image, etc of him could be appeared and blizzard purposefully named it that to throw people off.
Where have they said that? Or is it all player assumptions. All I recall is them saying it's time to take the fight to the Legion and try to end the threat once and for all.