All the lore is very well known by the D2 community, the best bet will be smth like "Diablo 3, The Horadric times." So as we all know, D2 left no room for sequels.
And i would kill to see the bone castle of Mephisto.
Also it would be good to see the Diablo and Baal at the top of their games with their original forms and a much younger Decard Cain.