Take this one with a pinch of salt, but there's a possibility that Diablo 4 could be revealed at this year's BlizzCon.
The news is coming via a French publication known as Le Monde. In a paywalled article discussing the layoff of employees in Blizzard's Paris studios, some employees apparently let the news slip that they'd seen an internal presentation of the new game.
Allegedly coming to console and PC, the titbit claims that they won't be released before 2020, but mentions nothing about a planned official reveal. Translated from French, the article said;
What about the future? Blizzard Entertainment teams have already had the right to a presentation of the highly anticipated Diablo 4 and know that a new Overwatch is in the pipes. But none will come out until 2020, at best, and the employees of Versailles do not know if they will see the color. "Activision Blizzard will make less profit, but still make a lot of profits," says one veteran.
Now, why we suspect BlizzCon? Well, there was a rumour for the last convention that a new Diablo game would be announced - and it was - a mobile game - which was very poorly received by those in attendance and the wider gaming world, forcing a rethink.
We'd expect, given the announcement of Project Scarlett at E3, that the game may be aimed at the next generation of consoles, so Holiday 2020 at the earliest, but this is purely speculation. Hopefully we do see official confirmation at BlizzCon, though, which will take place this November.