Remake will be somehow "inside" Diablo 3 as a set of levels. Graphics will be based on the engine of Diablo III, however, the manufacturer has announced that they will apply special graphics filter to render the archaic atmosphere of the game.
The player will be able to move only in eight directions, though the exact nuances of the game are not yet known. In the game will appear soundtrack from Diablo 1, as well as the opponents.
And as Frank Pearce, co-founder of Blizzard, said at this year's event for the fans – BlizzCon - there will also be a lot of secrets to discover.
The expansion called "The Darkening of Tristam" will be available for free for those who bought "Reapers of Souls", the first expansion for Diablo III, both on PC and consoles. I think that most of the players already have bought this expansion, so it should not be a problem.
Be sure to record or stream your gameplay when it comes out. With new Action! 2.0 of course :)