What if the next Diablo game were a "prequel" based around the Sin War lore? The flavor of the game would have to be centered around eternal PVP, if it were based on the Sin War. You could play as Angels and/or Demons.
But what could the game format be? Blizzard already has a MOBA with Heroes. Overwatch fills the first person, team-based competition genre. Would it have to be the same 3D isometric display as existing D3, but the game engine built entirely around PVP instead of PVE?
What if it were a persistent "sandbox" world? Players log in and log out, but the same battle is forever being fought?
Not sure how the progression system would work, if there would be leveling/gearing/skill trees, etc.
This idea is obviously off the cuff so feel free to shoot it down in flames, lol.
This story always seemed the most likely route given the amount D3 developers that have been cannibalized for games like WoW. I think the folks expecting a big D3 expansion are gonna be so disappointed after gamescom and blizzcon.