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