I am certain that the next expansion will be Legion-themed but I don't think we're ready to go to Argus yet. Perhaps the Legion stages their assault from Xoroth under the command of Tichondrius? We go to Xoroth to finally kill the Nathrezim who we couldn't kill on Azeroth (because they need to be killed in the Twisting Nether, right?). Other villains could include Balnazzar, Varimathras and Anatheron; plus multiple other demonic villains.
It could have the same eco-diversity as Outland, neutral playable Etheral (who had fled to the planet when Dimensius , Demon Hunter class and the return of Illidan. Maybe with Illidan's return, Maiev is on the hunt again, except this time we're against her. And maybe Xoroth is the world where Alleria and Turalyon are holed up?
What do you guys think?