Diablo 4 seasons will overlap with expansions like the Vessel of Hatred

gamesradar.com:

With Diablo 4 embracing a live service structure for the very first time, it's difficult to know exactly how Blizzard will handle the release of key expansions. Associate game director Brent Gibson sought to provide some clarity at BlizzCon 2023, telling me that "we're not going to pause" the seasonal structure when Diablo 4: Vessel of Hatred launches next year. 

As it stands, Diablo 4 is cycling through new seasons every three months – each offering unique game mechanics, endgame activities, and rewards tied to free and premium battle pass tiers. Since launch, we've had the Season of the Malignant and the Season of Blood, and these sort of themed seasons are set to continue in perpetuity. Even when the first premium expansion launched, which is set to whisk characters (including a brand new mysterious class) sequestered on the Eternal realm off to a brand new region of Sanctuary to continue the core storyline. 

"When we launch, we're going to have stuff for Seasonal players, we're going to have stuff for the Eternal players… Basically, every player type that we have, we're going to have something there for them to play with. That's really important to us," Gibson continues. "There's so many playstyles out there right now, and we don't want to alienate any of the groups along the way." 

That means that whether you decide to buy the Vessel of Hatred or not, there will still be a new season of content there for you to dive into. Whether the battle pass rewards will be heavily inspired by the new region and class remains to be seen, but for the time being it's good to know that Blizzard is keenly aware of the risk associated with dividing the player base through new expansions – expected to launch every year, rather than