From what I've hear'd Blizzard is slowly working on this in the background for a long time now. It would be a massive project to make this happen as they have to redo the entire old world, making borders, preventing glitches and getting stuck etc.
Ganking would be worse and it's kind of cool to have the big old world with unreachable places imo. Keeps you wondering
Griphon rides are the perfect moment to grab a drink aswell
I see.. that would bring more users on a massive level to this game. I was just thinking of it.