Timezones can be annoying to figure out, so to simplify: it'll be live when this comment is 8 hours old.

Edit. It's on now but they're going on about Immortal instead of d4

Edit 2. Immortal bullshit still going on

Edit 3. patch today, helltide chests can drop uniques

Edit 4. Season of Malignant / S1 talk going on

New story stuff related to season i.e. new boss, events "beyond" main story. Season starts in Kyovashad

New powers

Elites can spawn as Malignant versions, killing them & using an item causes a more powerful elite spawning - beating tast stage elite gets you a socketable malignant heart "gem" granting season specific powers.

32 malignant powers to be available.

Only fits to infected sockets, 3 colored sockets to match, 4 heart types with 1 being prismatic/fits all

Higher world tiers - Stronger hearts

Specific dungeons, malignant tunnels, with more malignant elites besides just ones found in open world. End of the dungeons lets you choose the colour of the heart / malignant enemy

New uniques and legendaries - Will also be added to Eternal realm, not just seasonal

July 18 season patch, changes available on eternal

July 20 season starts, realms open and charactet creation available

Battle pass and season journey with 7 chapters

Item and aspect rewards (some for codex of power that aren't available as dungeon unlocks)

Season rewards favour which progresses battle pass

Chapters can be "complete" without doing everything, f.ex. you dont have to do pvp

Everyone has access to battle pass and seasonal journey without paying.

Future seasons might start in other cities/hubs

Battle pass rewards include cosmetics, horse armour, new emotes. Lots of free stuff, but you can pay battle pass upgrade, gets you just cosmetics

Unlockable boost stuff for alts in season

Edit 5. I gotta go so anything past this point, not gonna keep updating

Edit 6. Okay back, missing anything before this up to edit5 point.

Seasons last 12 weeks / 3 months-ish

All season stuff here: https://diablo4.blizzard.com/en-us/season