I doubt it will be an MMO. They can't afford to cannibalize WoW subs. I doubt it's going to be a traditional Diablo game either. Looking at HS, HoTS and Overwatch they are all riddled with monetization stuff. I'll guess an episodic game or something more like D3 with seasons that have enough new content each time they can charge for access to each one. My longshot guess, Diablo battle royale. Though I think an overwatch engine based battle royale is more likely.