Rod Fergusson, General Manager of the Diablo franchise, has confirmed in an interview that Diablo 4 will get annual expansions. Fergusson already commented on Blizzard's plan regarding expansions for Diablo 4 around the game's release, saying that two expansions are already in the works.
Being asked about the actual length of Diablo 4 development in an interview with Dexerto, Fergusson commented the following:
Dexerto Interview Multiple D4 Expansions in the Works
It is refreshing to see the shift in design philosophy, especially compared with Diablo 2's and Diablo 3's expansion «cycles.» The fact that Diablo 4 is about to see one expansion each year in active development would certainly cement its place as the live-service game Blizzard has been promoting over the last months.
However, the community is already voicing concerns about the development team's bold plans, criticizing the fact that players will need to buy full-price expansions every year to unlock fresh content like new classes, while others hope for regular extensive and meta-changing content drops full of «new systems and reworks.»