It's not like blizz to put a bow on everything, expect some curve balls. :)
First of all I think Deckard is hiding an ace up his sleeve. I highly doubt he's just a feeble old man. He's managed to control our path since we descended catacombs to defeat a mad king. Not to mention he's freaking old...http://us.battle.net/d3/en/forum/topic/3082081632
As far as the prime evils go... JW said that the events have transpired exactly how they planned. So to say they're erased from existence is a little premature. What better way to leave the public eye than to 'die'.
The Nephalem are finally starting to rise to their potential after the destruction of the world stone (this is apparant in the new skills.. wrath of the berzerker, shadow power, ect). It seems highly unlikely that angelic/demonic forces have left sanctuary alone. It'll be interesting to see how they've been manipulating us over the years.
With the world stone gone (with it the pact with hell to leave sanctuary to grow without interferance), I wouldn't be surprised if the Angiris Council has decided that the Nephalem have outlived their usefulness.
Bare in mind we have 2 xpacs coming off this title so we won't get the full story at launch :(