Diablo 2 remake? Diablo 4? Diablo MMO?
Everything is on the table.
So what could most likely happen?
Diablo 4 - Unlikely. Diablo 4 would struggle big time to compete with PoE. They dropped the ball so bad with D3, that fans are definitely not quite trustful on this one.
Diablo 2 remake - somewhat likely. It seems that appetite for remasters is still quite going and It wouldn't surprise me a bit to see a remake. However Diablo 2 is still shallow in comparison to PoE. While it definitely attracts veterans, new fans will likely to stay away.
Diablo MMO - highly likely. Blizzard needs to start making decisions. WoW is dying thanks to paid subscription. And they clearly running out of ideas for new expansions. I would see WoW going free in near future with paid expansion here and there coming out and game ridden with even more micro-transactions just to keep this beautiful world alive while working on the next big thing that will reignite appetite for MMO's.
http://www.denofgeek.com/us/games/diablo/274007/diablo-new-game-coming-from-blizzard