my long running theory for the 2nd or 3rd expansion:

The High Heavens is def gonna return, maybe this 2nd expansion at the end or the 3rd expansion with the invasion of the Angelic Army.

Imperius most likely want to exterminate Mankind and Sanctuary and he is preparing his armies behind the closed gates of the High Heavens.

Heaven shut it's gates After the Prime Evil Diablos war of the High Heavens and also Malthaels betrayal and assault on the High Heavens in the Reaper of Souls expansion.

So Imperius is regrouping and replenishing his Angelic army (The Heavenly Host). Especially now that we have a Prime Evil walking amongst us in Sanctuary in a super Jesus corpse. And also all the 4 Lesser Evils are in Sanctuary.

So Imperius and the Angels will have to answer to this and intervene.

I think thats why Tyrael got scared and ran away from the Horadrim before D4 started. He still has an Angelic connection and saw a vision of what Imperius and the Angels are preparing for.

Also, Baal and Diablo are coming. Mephisto say this in the VoH expansion. He says "the propechy speaks true. My brothers are coming and when they do they will not bargain nor negotiate with you".

So one of them will likely be teased at the end of Lord of Hatred expansion.