I had thought I touched on the subject previously but apparently I did not. A Diablo-based MMO? Well I can see it but there are several reasons why it wouldn't be as viable an option in its current state.
Factions: Currently there aren't really any factions in the Diablo Universe. The game is solely good versus evil. That would make a one-sided MMO which, in my opinion, would be counterproductive. Look at World of Warcraft, Alliance against the Horde. Humans against Orcs. That makes for a good conflict, what would PvP or hostilities be like in a Diablo MMO? Arenas. It would be a Massively Single Player game in it's current state.
Races: None. You are a human and that is it. I could see a half-demon half-human race springing up from years of raping and pillaging on the demonic side but in the current lore there is only one side that is playable. That eliminiates one form of customization.
Classes: Well the classes are defined and truely the Diablo 2 talent trees gave rise to World of Warcraft. I think of WoW as a hybrid between Warcraft 3 and Diablo 2. Barbarian, Sorcerer, Necromancer, Druid, Assassin, Amazon, and Paladin. Those classes would work but they're already defined (mostly) in WoW. Warrior=Barbarian, Sorcerer=Mage, Druid=Druid, Necromancer=Deathknight, Amazon=Hunter, Paladin=Paladin, Assassin=Rogue.
The World: The Diablo universe has not been fully defined. You have no world, no continents just cities (unless I missed something, which is possible). Without a larger world there is no MMO, there is just a Massively Single Player game.
Making a Diablo MMO would turn the world upside down in its current form. Expect Diablo 3 first, and if Diablo 3 changes the lore or updates it in a large way then expect a Diablo MMO soon after. Like I said it isn't that it's not possible it's just not feasable without a few changes.