A bit of info from the early development of the next expansion:
World Revamp. There will be no whole world revamp, but a chunk of zones at the start of the xpac and then other zones will be added with patches.
- First bunch of the revamped world is whole Lordaeron and Wetlands. Gilneas, Menethil Harbor and Silvermoon all will be proper cities. Arathi will be same as in BfA.
Level Squish. Amount of levels will be squished and currently planned that there will be no more levels added in expansions after that.
Dynamic Zones. New and revamped zones will be less level-gated and you can do content in the updated zone both with your BfA character and with a new one.
Events. Evolution of World Quest and Assaults system. There will be events in all zones after the event starts you will have accesses to a unique set of quest and zone objectives. One zone will have a set of different events and more will be added with patches for all zones.
Patch Cycle. Xpac planned to be much longer with a big amount of patches.
Gameplay. Class reworks is in the work. Also, rework of a few systems like GCD and auto-attack.
Races. Allied races will have slight rework to setup pattern for the addition of more unique races.
Story. There will be no one big storyline for xpac, but quite a few different ones and patches will be dedicated to different themes.
- 9.0 content will be centred around Lordaeron.
- There will be the continuation of BfA storyline, but much more in background.
- The bulk of the story will be centred on different races and in-factions fightings.
- Horde will be struggling to decide that leadership it wants.
- The Alliance will be on the brink of dissolution because different leaders will have conflicting goals.
The codename of xpac is
"The Dark Age"