I don't play a Warlock, but the entire quest line plus reward of green fel fire sounds really cool to me. I hope that they add more of these class specific quests in the future. What events and rewards would you like to see for a particular class?
Priest - Reward "Replica Benediction" for Transmog
- could include a questline similar to the original Benediction quests requiring you to heal multiple solo events and dungeon encounters
Rogue - Reward "Ability to Display Poison Dripping from Weapons Instead of Enchant"
- could include stealing, poisoning, and assassinating multiple NPCs throughout Azeroth