I would like to see more randomized dungeons, deeper story telling and more epic cinematics.
I would like better character develop-ment, IMO it wasn't enough
More Easter eggs
As for classes Druid(obvious) and of course(for me) Necromancer
Let me justify Necromancer:
Darker feel than witch doctor
pets would be long term as opposed the the WDs short term
Could have debuffs(for enemies) and buffs(for players)
The corpse based spells will be very nice to have back, as it adds a very distinct feel for a class
and at last bone spells, they wouldn't be very unique in terms of what they do but some people prefer a more "gothic/skeletal" aposed to a holy, tribal, arcane-ic, brutal, and archer/theif style
I, like many others, who used to play necro don't get that necro feeling when playing a WD and bringing back a necro would make a lot of people happy.
EDIT: I Would also like some garenteed drops(a class rare or somthing) on first kills of bosses through out the other difficulties not just the first, I found my self never finding gear for my wizard after normal, and even after farming, resulted in me going to AH jsut to get through the game.
Yes I played wizard as a main instead of a WD, I don't like the feel of a WD i like the feel of a necro :-P.