Many have asked, and now with the launch behind us, we can lay out the progression timeline for the rest of 2020.
There's a long road ahead for leveling, attunement, and raiding. Since all content is currently open from heroics through Kara, Gruul's lair, Magtheridon, SSC and TK, patch 2.1 is currently scheduled for late July. Patch 2.1 includes the Black Temple and multiple vial drops for Mount Hyjal, as well as the Netherwing, Ogri'la, and Sha'tari Skyguard dailies.
The next major content patch after 2.1 is 2.3, and features everyone's favourite: Zul'Aman. Patch 2.3 is currently penciled in for early November, which will ride out the end of 2020.
For those of you interested in PvP, arena season 1 will kick off Wednesday, April 1, 2020. Future arena season changeovers will follow approximately 1 week after the major patch releases.
2.4 and the remaining arenas will take place in 2021.
So in brief:
Late July - Patch 2.1 - Dailies & Black Temple
Early November - Patch 2.3 - Zul'Aman
April 1 - Arena Season 1 Start
1 Week after 2.1 release - Arena 1 End / Season 2 Start
1 Week after 2.3 release - Arena 2 End / Season 3 Start