https://kotaku.com/the-past-present-...blo-1830593195
Kotaku (and their reporting, especially on Activision Blizzard, is usually spot on) stated last year that Diablo 4 was a Dark Souls-eque game but was canceled and rebooted in 2016. The rebooted effort is much closer to a traditional Diablo game and is being built as a GaaS. Some insiders stated that the game was supposed to be announce at Blizzcon last year, but was pulled. Other insiders have contradicted this.
Kotaku believes the failure of Titan is causing Blizzard to not announce games until they're closer to release.
Maybe we'll see it at Blizzcon this year. Maybe we won't.