For those wondering about an offline mode, not happening as it will be always online (like D3)
"Diablo 4 will not have an offline mode and will require an internet connection to play, Blizzard said during an interview at BlizzCon today.
We asked lead designer Joe Shely if it was possible to play Diablo 4 offline. "There's a large, seamless, connected, and shared space in the world, going down into dungeons, being able to group with your friends, trading and PvP. We feel that the best way to experience that is in a world that is online," Shely responded."