Blizzard added an all new Crusader class for its first Diablo III expansion pack, Reaper of Souls, so clearly new classes would need to be added in future expansions. It would be good to see the return of a shape shifting class in the vein of the Druid class introduced in Diablo II. A Daemon or Necromancer class with the ability to change into various forms would provide a completely different playing style to those already on the roster.
In would also be great to see a return for the concept of rune-words which were an interesting addition to Diablo II. A new Diablo III expansion could expand on this by having more specific rune ordering which would be able to turn a standard or rare item into a completely unique legendary item should the runes be attached in the correct order.
Following on from the addition of 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 on Earth and fight against the kind of foes which made the first stages of Diablo III so much fun to play.