If there's one thing I've learned it's that popular features will be one-and-done while Garrisons live on.
Cynicism aside.
I have to imagine we'll see world quests even while leveling in 8.0. Blizz tinkered with the idea with the Legion invasions and I think it does a TON to vary up the leveling experience.
Class Halls/Garrisons will just get copypasta'd over some new skin. Probably boats or a shipyard 2.0 since it's gonna be Kul'tiras/Azshara/N'Zoth/South Sea next.
Artifacts is the system I'm curious about. Blizz has all but confirmed our artifacts will be put in the bank after we hit 111, but the idea of an alternate advancement system, even at max level, is an idea I've advocated for a LOOOONG time. Maybe we get a new talent-esque, artifact-esque system that kicks in at 120?