Diablo 4 isn't due for release until next year, but in their latest quarterly update Blizzard have laid out their plans for post-release updates. Those plans include seasonal content, a paid season pass, and an in-game cosmetics shop. It's everything you'd expect (or exhaustedly resign yourself to) in a live service game.
"Diablo IV's Seasons are modeled after those of Diablo III," says the latest post about the action RPG on the Blizzard blog. "When a new season begins, all the characters from the prior season are moved to the Eternal Realm, where you can keep playing, leveling up, and collecting loot. To play in the new season, you’ll create a fresh character and experience the new seasonal features and content while leveling up alongside other players."
Each new season will have "a fresh new gameplay feature and questline that introduces new challenges, mysteries, and possibilities into the level-up experience." There will also be time-limited live events, incentives to re-explore the world or retry old classes, and a paid season pass. The seasons will be available to all, but the season pass will let you advance through Premium Tiers of a progression track, with greater rewards than are available on the Free Tiers. Paying will "provide no in-game power or advantage over other players," says the post.
The same is apparently true of the in-game shop, which will offer cosmetics to let you alter the appearance of your character. "Nothing offered in the Shop grants a direct or indirect gameplay advantage," says the post. The shop will have some exclusive items, but a lot of them will also be available to unlock via in-game drops. Perhaps most significantly, the post states that "the shop is transparent", with players knowing what they're buying before they buy it, which sounds like a round-about way of saying they won't have loot boxes.
Blizzard have received a lot of criticism recently due to the monetisation of the free-to-play Diablo Immortal, a game which doesn't feel at home on PC and on which you can spend tens of thousands of dollars. While live service games immediately exclude me, Diablo 4 and its upfront cost does sound a lot more reasonable.