During BlizzCon 2023, Blizzard revealed the first expansion for Diablo 4 titled Vessel of Hatred. Details were sparse, but the expansion will focus on Mephisto, one of the three Prime Evils. The expansion will take place in the Jungles of Torajan and feature a brand new class.
According to Blizzard the expansion is still early on in development and more information will be available in Summer 2024. No firm release date was provided, but the brief trailer revealed a timeframe of Late 2024.
Timing wise it makes sense, as Blizzard confirmed earlier this year that Diablo 4 will receive annual expansions alongside its quarterly seasons. This likely means we will have another two or even three seasons before the Vessel of Hatred expansion releases.