I still want a Dark Souls style Horadrim origin RPG.
Bring it back to a slower paced dungeon crawler. Set it during the Horadrim at the height of power, hunting down agents of the prime evils and their eventual banishment. Set it so far back that Deckard Cain doesn't even get a mention, showing the history and introducing disciplines we've never seen before that were lost over the ages.
All the classes can be more reclusive and archaic forms of what we have today. Maybe give it a more victorian inspired feel, as if the Horadrim had access to technology that was way more advanced than it should have been, but all information lost by the end of the series. It should feel a lot more gothic horror with every member being a mere mortal accessing greater powers for the first time.
This would be a far cry from the lootfest of a D4, but I think we need something to get us back to the spirit of what Diablo was all about. Exploring random dungeons with threats coming from all corners. No running around, just walking and trying to survive. No skills from leveling up, you need to find arcane sources or ancient secrets to empower your weaponry. Think of participating in crafting some of the greatest legendaries we use today. I think having this in between D3 and D4 would be a good pace-breaker and a great setup for potential story hooks for the future.