The Necromancer is being added to Diablo III. Inspired by the Diablo II Necromancer, it will be available next year as part of a paid package that will include other stuff (though we don't know yet what.)
Additionally, as part of a free patch, the original Diablo game will be added into Diablo III - you'll be able to take your Diablo III characters through it.
The Diablo I section will have a visual filter that pixelates things to look more like a 1996 game and will limit movement to 8 directions for true authenticity.
So nothing about a new Diablo game or expansion, but I'm SO ON BOARD with getting the Necromancer, even if it does sort of step on the toes of the Witch Doctor. Xul is my favorite Heroes of the Storm character.