Quote from Bagstone3) Revolutionize the game. This comes at a cost: we'll leave the ARPG genre. Some might like it, but we'll without a doubt lose some people. Just like Fallout became one of the top games with its re-vitalization by Bethesda, the game is now super popular and has certainly made great strides forward. I personally got lost - FO3/4 aren't for me. That's fine. I have to accept that a Fallout 2.5 is something that is too niche. Just like Diablo 2 Remastered would only be played by a handful of people (if you think it would be a game for the masses think again; StarCraft Remastered has shown us how you're in for a rude awakening once the nostalgia is gone, which was the case for 90% of people). Diablo 4 could be a mobile-based game like Pokemon Go. It could be an RPG and get rid of the action part. It could become a card game full of microtransactions (oh wait, there's already Hearthstone). Or it could be something entirely new no one is thinking of!
I've always been intrigued by the idea of introducing the "holy trio" into the Diablo franchise. The fast paced, constant slaughter of demons with the added strategy of the Tank picking up specific enemy types to keep them from one shoting the DPS. Healers weaving in and out of mobs while trying to maintain the health pool of his fellow adventurers and DPS racing to send the demons back to hell before members of their party get sent there first.
I've seen this idea tossed around before, and it's always been met with mixed reviews. But I would certainly enjoy seeing something like this introduced into the Diablo world.