One possibility for the next expansion would be for the mortal races to take the fight to the Burning Legion on the Man'ari Eredar homeworld of Argus.

This follows the logical progression of largest "bosses": Illidan could not beat Arthas, so we met Illidan in Outland before we meet Arthas in Northrend; the Burning Legion created the Lich King, so after seeing the Lich King we would see the Burning Legion.

Two of the worst demons to plague Azeroth were Archimonde and Kil'jaeden, both of which are eredar and come from Argus. Since only the avatar of Kil'jaeden was defeated, it may be possible to meet him again in the flesh.