Here's a class Idea: The Heretic.
The Heretic is an INT class that cuts his wrists to use his life as a resource and is heavily reliant on lifesteal. Spells include setting his blood on fire, enslaving demons, exploding heart spells, and demonic shapeshifting. Basically a bit of Druid and Necromancer with a unique and unsettling twist.
The Act should take place in hell. I was disappointed that the interesting depictions in the Book of Cain were absent. Don't get me wrong, what we got was very good and very much reminded me of D1 Hell but I'd like to see more than just Fire and Brimstone. Also, was The Core of Arreat even hell or just an outpost?