My guess is that if a D4 ever comes, they make it the same style of multiplayer as PoE, with shared hubs, chat, guilds, and the like.
I don't know if it would be more popular though. Blizzard has shown an unwillingness to develop individual seasons with D3, and with no effort put into seasons development-wise, players return the favor.
Blizzard missed a big opportunity with D3, and as a result have been replaced entirely by a new game that is gradually becoming a bigger and bigger thing. At the rate PoE is growing, I don't even know if a D4 could remain competitive.
as long as wow is up and running i really doubt they will release another MMO as that could potentially cripple both games' sales/monthly fees. on the other hand, if wow is about to end then there is no better game to replace it than a diablo MMO. Diablo MMO makes perfect sense the way they finished ROS.
I seriously hope they keep Path of Exile and World of Warcraft out of Diablo!
If I wanted to play an MMO, I'd be on one of those - but I hate them both with a vengeance!
WoW is only team oriented and has monthly fee, and PoE is geared towards hardcore no-lifers.
I don't mind the option of PvP, guilds/clans, micro-transactions and teaming up; so long they don't define the game and become the only means to earn meaningful rewards.