At the 2016 Blizzcon event, Blizzard Entertainment announced that the original Diablo game is being remade inside of Diablo III. In a special 20th anniversary update called The Darkening of Tristram, the patch will contain a sixteen level cathedral map and all four bosses from the original game. The patch will be available on the Public Test Realm as early as next week for testing.
No other further details are available at the moment for the The Darkening of Tristram patch.
In addition to the remake, Diablo III will be receiving the new Necromancer class as well as two brand new zones for the Reaper of Souls expansion. The necromancer class is coming to Reaper of Souls expansion sometime in 2017. The class is currently available to play if you are lucky enough to attend Blizzcon