"After the death of Malthael, the combined Prime Evil Diablo was released, rather than the individual Evils" http://www.diablofans.com/news/48075-more-info-from-the-anniversary-dev-stream-gambling
Tyrael also says, after Malthael is defeated that Diablo is released, not giving a sense of all of the evils, but Diablo as a singular enemy.
Are we really only going to face Diablo from now? Never again the other evils he combined with like Mephisto, Baal, etc ?
I'll be honest, I will be pretty disappointed if it's just Diablo from this point. Part of the fun in Diablo lore wise for me, is it wasn't just a singular leader of the Infernal Hells, but a large army lead by numerous badasses.
And with Heaven not really backing us, the odds seemed so overwhelming against Sanctuary and it's inhabitants, which made for a dark and interesting challenge like "How can we possibly win against all these awesome enemies?"
But now Heaven is more or less supporting Sanctuary, and only one enemy of Hell to defeat (Diablo as the singular, Prime Evil), which we already have defeated, is this still a challenge? Even without Heaven's support, the Nephalem is more than capable it seems to defeat Diablo again.
I for one would welcome the return of ALL the evils, traitors banished (Inarius, Lillith for example), and whatever else can be thrown at us. Otherwise, I don't see Diablo as a game franchise, lasting longer than another xpac, or even being that interesting in the next.
Sorry to sound pessimistic, I just love the lore, characters and want to feel like there is still a lot more to come story wise for us. Like Warcraft and Starcraft, there is SO~ much Blizzard can do with the lore in game and I hope they don't fall short of reaching out to deeper and more challenging adventures for us in Diablo.