Blizzard added an all new Crusader class for its first Diablo III expansion pack, Reaper of Souls and it would be good to see more than one new class added in future expansions.
It would be interesting to see the return of a shape-shifting class in the vein of the druid class introduced in classic Blizzard game, Diablo II. A necromancer class with the ability to change into various forms would provide a completely different playing style to those classes already on the roster.
Following on from the addition of a well designed Act V in the Reaper of Souls expansion, Act VI would see a return to the graveyards and old ruined cathedrals as seen in Act I. The player would venture deep into the oldest catacombs and cathedrals and fight against the variety of enemies which made Act I of Diablo III so much fun to play.