Demons don't work quite the same way in Diablo as they do in the Warcraft universe. They're not likely to bend to the will of a mortal. Also, the gameplay of a Warlock would be too similar to Witch Doctor if the focus is on pets, and too similar to Wizard if the focus is destructive spells.
I think the most likely, and most different class they could (re)introduce would be the Druid. Other than that I wouldn't be surprised to see an entirely new concept, like the Blood Mage suggestion that was posted some months ago.
And I am just here with the unpopular opinion, that they should begin working on Diablo 4 instead of a new expansion, learning from their mistakes and hone it. Preferably bringing back a more sinister color-scheme, that gave so much more to the atmosphere of the old game.
But if it HAS to be this way, by guess would be that they implement some sort of Necromancer or Druid, in whatever form that might be.
I think that they need one more expac to setup for diablo 4. I would be okay if they didn't create a new class though.
Do we really need a Diablo 4? This game has been moving in the right direction every patch since 2.1.2 in my opinion. I think it's getting to the point the game could really get some staying power. I think if they added in runewords, added new weapon effects for gems, and maybe added 2-3 more gem types that would be awesome.
The problem is Blizzard specifically stated they wouldn't reuse any of the old classes. At least not directly. Arguments can of course be made towards the Crusader basically just being a Paladin.
If they were to introduce the Druid there is also a bunch of other issues involved, like the fact that the iconic Werebear ability is a concept already used by the Witch Doctor, although not in a shapeshifting sense. The beast pets, like the Wolf and the Ravens are also used by the Demon Hunter. Twister, Tornado or Hurricane-esque abilities, and Molten Bouder or other elemental abilities are similarly used by the Wizard and the Witch Doctor.
If they were to find some loopholes to re-introduce it (introducing the.. shapeshifting Shaman!) and manage to capture the essence of the class without simply rehashing other classes abilities, and if they were to make a shapeshifting system that works and is fun, I'd be eager to test it out though.
Actually, one of the Diablo concept artists created a bit of Druid artwork some while ago. So maybe?
I like the idea of a blood Mage type class, but having vitality as the class's primary stat would allow for an extra stat slot which I think would be a little bit too strong. Also I still think they need to make a ranged Strength class, and a melee Intelligence class with 30% damage reduction.
I think we should take the golem part of the necromancer and turn that into its own class. Allow it to use earth, water and wind based spell. Maybe call them a Geomancer or something. Just spitballing.
The barbarian is the only returning class. Originally we actually planned to have no returning classes, but as we developed the barbarian, it essentially ended up turning into a barbarian. We sort of got to the point where we went – because we actually had a different name for him originally - I think were going to call it this other name and everyone else is just going to call it the barbarian, so maybe we should just call it the barbarian. One of the things we really did not want to do – it's why we set that initial goal to not repeat classes – was, we didn't just want a rehash; we wanted to make the sequel and the sequel has new gameplay and new experiences. I know a lot of people really love the classes in Diablo 2 and it's not my desire to intentionally arc those people – I like those classes to; I love the Necromancer, he's my favorite class from Diablo 2 – but as our goal we really wanted to provide new experiences; that was our focus.
I think a few of you are on the right track, but maybe the class needs elements of a lot of these idea. Call it a blood mage, but vit as a resource sounds clunky. Have it be an Int character with a mix of ranged and melee capabilities kinda like the crusader, only where the crusader is all righteousness the blood mage takes a fight fire with fire approach and is as bad as he has got to be. Blood is his resource, some of his own, but mostly the blood of his foes. the possibilities are endless. there could be demon skin/limb grafts giving him crazy powers or forms. maybe reanimated demon corpse mech suit/ mount that he fights from. He could meld into darkness for a stealth or rogue like approach to things. Maybe he can jump into demons and other beasties and take control of them. I think there are a lot of fun possible options for a class like that and it could bring some of that darkness, us long time diablo fans love, back to the game.