Honestly I feel like they need to compare the success and the not so great features between Diablo 2 and Diablo 3 and begin to try to incorporate what worked well in the past. There are SO many features that were fantastic that haven't been brought over that could be great expansion material such as runewords, cube recipes, skill trees, pvp, stat selection, etc.
Also, there really needs to be some sort of end game content beyond farming items. Perhaps some sort of side games or anything besides logging on and doing keywarden / MF runs.
Although more classes for sure would be fantastic, I strongly feel skill trees and stat selection much like Diablo 2 could open up so many more possabilities with character builds. Currently its VERY cookie cutter. In the past, if you had an assassin it could either wear caster gear and be a trap assassin, or wear melee gear and be a martial arts assassin. A druid could use a werewolf melee build, get strong resistances and make a bear build, or seek out caster gear to have a hurricane / elemental build. Now, for any of the classes, yes there is a little variety but its the same gear same play style. Yes classes in D3 are more user friendly than D2, but thats what made D2 so much more engaging, by allowing people to really build and establish how they want to play each of their characters.
As far as plot there are several routes to be taken. Adria's story and a witch hunt for her, Imperious' hatred and perhaps jealousy of humans growing, the true nature and corruption of the Templar Order, tensions with the Westmarch nation, the thieves guilds and their dealings.
If it isn't broke, don't fix it. If Blizzard wants to repeat the huge success they achieved with Diablo 2, they really need to look back and see how they accomplished it before to realize how much potential Diablo 3 has today.