Multiboxing-like system where a player can summon his own self-created alts on the same account - controlled by an in-game built-in multibox-like AI. Limited to 2 alts maximum. This is the next step in the evolution of the body guard system. This will be limited to leveling areas only with pvp /warmode mode off. With this system, 2 players are enough to run through a dungeon in normal and heroic modes. Not allowed in mythic and m+. This would also encourage players to once again want to quest in the outside world as opposed to dungeon grinding.
It also serves as a direct competition to FFXIV's multi-class system via a single toon. This is more exciting even though limited to 2+main toons because you get to play them all at the same time.
Another feature that was also moved to 10.0 is secondary specs. A mage, for example, can be primary fire and secondary frost. Basically kind of like in the classic trio but much more limited.
Yet another one is all gathering professions are given to everyone while crafting staying one per character. This is how they intend to flood the economy with matts to reduce the amount of gold players can make so they would have to pay with real money as opposed to with gold. At the same time, it encourages people to craft their own stuff more often.
One more feature that got moved is making use of 15 years worth of content as filler content, instead of mindless grinding of reps and making players quit faster than ever, they finally are going to create incentives to go back to older stuff and actually enjoy that content in new ways. Kind of like reusing Org and SW in 8.3 as an endless grind in 1-5 flexi mode.
Aerial combat is forever in development hell being worked on as part of an engine upgrade.
Heavier use of cash shop in the future to support housing / casino and other new cosmetic-only systems.
Instead of mobile WoW, they are making the battle pet system available on mobile, allowing players to level up alts via pet battles from their mobile phones. Can play versus players and do pet battle dungeons on the go!
The reason these got moved to 10.0 is because 9.0 already has enough new and exciting systems (well, in the eyes of Blizzard, that is.)