My wife and I are both longtime players of both Diablos (refuse to acknowledge the cartoony trash of III). Our suggestions:
1. Hellfire level! This was a cool, creepy mod in Diablo I that was missing in II. Yeah, the cow level is cute, but nowhere near as gross/bizarre. A portal in town to Hellfire would be sweet.
2. Better arcane/magic attacks: holy/arcane/magic damage isn't great, and could use some tweaking to be competitive. Could tie nicely to the higher level skills already in dev.
3. Blood magic for necro: you could have blood fountain, blood wave, fist of blood, etc; would look really awesome and add a whole new subset to the character (why is it just poison and bone?)
