Diablo 4 has two expansions cooking already

pcgamer.com:

Diablo 4 is about to break into the wild following a brief early access period, and the early reviews are pretty great with multiple perfect scores (PC Gamer's review is still in-progress, and will be finalised when we've had enough time with the launch game). An omnipresent aspect of Diablo 4's identity is the live service part, with Blizzard able to do things like hotfix a bug that made multiple elite enemies spawn on some poor souls (arguably worth it for the name 'Darcelpocalpyse'), and focusing on delivering a stream of substantial content for players.

Diablo general manager Rod Fergusson gave a wide-ranging interview to Kinda Funny about the game's early reception and, while discussing the live service element, casually mentioned that the team is now working on not one, but two expansions for the game. Fergusson said that the working process for Diablo 4 had to take account of the fact the game's development is intended to be a marathon rather than a sprint, and Blizzard didn't want a situation where the game launched and the team who made it was burned out.

«So you have to build your team and your structure around a way you can do that sustainably, because you're kind of always shipping in a way,» said Fergusson. «As I sit here we're about to launch the main game, we're finishing up season one, we're working on season two, we're working on expansion one, we're kicking off expansion two… we're working on that now [laughs] and we haven't even launched the game.»

So two expansions in the works already and, based on the general vibe of how Fergusson talks in this interview, expect them sooner rather than later: He says one of the things they look back on with Diablo 3 is that perhaps Blizzard didn't deliver enough