From what I've seen and read so far it seems D4 will include MMO mechanics. There will be worldbosses, and you can run into other players. Friendly and hostile town spread over the map. This will not be a classic ARPG, though I still hope the storyline will be of the typical not too shabby Blizzard kind. For a more back to the roots experience I reccommend Wolcen Lords of Mayhem, currently in beta and set for release in Q1 2020.