An interview with the developers of Diablo IV appeared on IGN's YouTube channel.
Here's What We Know
In this video the Blizzard's gameplay designers said that they put a lot of efforts to keep the players in Diablo IV even after finishing all the missions. To achieve this goal they've been developing the game in such a way, that it was possible to add new content for years.
The developers emphasized that even in the initial version of Diablo IV there are a lot of different activities for high level players. They aren't ashamed of loud expressions and say that players will spend not hundreds, but thousands of hours on Diablo IV and won't get tired of the game!
The example of Diablo III shows that this idea is quite viable, but the developers want the next part of the endgame content to be more diverse and complex.
We're not sure if Diablo IV will be supported, but we're getting closer to the official release date.
When We Can Expect It
Diablo 4 is scheduled for release in 2023 on PC, PS4, PS5, Xbox One and Xbox Series.