What might be behind it
This suggests that one of the new Forever dungeons will be another mega dungeon similar to Blackrock Depths. Blackrock Depths is a level 52 to 60 dungeon, and together with the somewhat spoiler-y picture in the tweet, it is likely that one of the two endgame dungeons is Blackmaw Hold.
The original Blackrock Depths has around 20 encounters and takes several hours to clear. Another mega dungeon in the endgame of WoW: Forever would add plenty of new content and rewards.
Original report: Wowhead