During the Blizzcon Developer Panel, Blizzard announced that WoW Classic Season of Discovery will have various phases with different level caps, before max level raids become the focus. Here is a breakdown of what we learned from various interviews, as the planned leveling phases --
Blizzard also stated that they will be watching players closely, to ensure each phase feels right for the amount of time before unlocking the next leveling phase. As of Blizzcon, Developers didn't give specifics on the amount of time, but hinted at it being more than one month, maybe two months.
Sneak Peek at loot available from Blackfathom Deeps
While Phase 1 will feature Blackfathom Deeps as the Level-Up Raid, Blizzard did tease some future content at the end of the Developer Panel, which showed an Alarm-o-bot (like ones found in Gnomeregan), a picture of Scarlet Monastery, and Karazhan Crypts. Gnomeregan is a Level 26-36 ranged Dungeon, and Scarlet Monastery is a Level 26-45 ranged Dungeon. While this doesn't give a clear picture, it's possible they might be both in Phase 2, or maybe one will have an increased level range for Phase 3?