they started with rogue legendary quests. they saw that didn't work well because the reward gets dated when a new expansion launches.
than they tried a total revamp on warlocks. I think it worked pretty well, now I see other warlocks besides me on random groups.
now they are testing the questline thing again, but this time using aesthetic rewards that don't get dated. the ring is not the main reward, it's only there to make sure not all warlocks will decide to plunder the black temple for money lol
my bet is on a rogue revamp and shaman class quests next expansion.