From what I heard, Blizzard will make one more Diablo 3 expansion. I thought of the imbalance one more class would bring, but what can they add that won't be a copy of D2 classes? I only came with the idea of Zakarum Priests being an Intelligence class, but then we would have 3 "holy" classes (alongside the Monk and a Crusader). But if Lorath - Tyrael conversation is a hint of the next expansion, maybe we will have a Druid class with a different setup. Maybe they will benefit the same from Dexterity, Intelligence and Strength. But then again, maybe not.
Vanilla Diablo was all about fighting the demons. Reaper of Souls is all about fighting renegade Angels. I would expect the next expansion to be fighting the other Nephalem. Considering the Worldstone has been destroyed 20 years ago, it gives plenty of time to have a band of Nephalem be born and raised with amazing powers and they want to be the rulers of the universe. Something like Zoltun Kulle. In that sort of fashion, and knowing Blizzard's storytelling, Angels and Demons will make an untrustworthy alliance and help the player defeat these new Nephalem (seeing as how he/she is the only one that matches their power).