I just don't believe that the hype is so low that it would warrant and make them gather and expend the resources necessary to create a new game from scratch. Thats why I believe that first they would revamp the game again with an expansion before commiting fully to a new game.
I dont think D4 is a good idea right now because it would lead to a D3 expansion with a Diablo IV label on it and thats not what anyone wants.
Diablo 4 needs to be a new game and not what Diablo vanillla is in relation to RoS