Something I would really like to see.
The main point is that every class would get a very demanding quest chain like warlock green fire quest chain to check if one is skilled enough.
After completing the chain you become a member of an elite organisation related to your class, for example:
1. warlock -> council of the black harvest
2. paladin -> argent crusade
3. shamans -> earthen ring
4. death knights -> maybe some council formed by kolthira, thassarian and orbaz?
When you finally are a member of the organisation you gain access to:
1. dailies with better rewards than ordinary ones.
2. quest chains involving some lore events
3. lore items (books, scrolls etc. so you can know about the org.'s origins and other stuff)
4. vanity items: mounts, 2 transmog sets(so ppl will have a choice - more variety), tabards and other stuff related to your class.
5. sparing with leaders(fighting kolthira/thassarian etc.) for nice rewards.
6. class-specific legendary chain.
I am aware that it would require a lot of work from blizz to implement but I would really love to see it in-game. I hunger for some more Black Harvest action as only 2 members were shown so far and generally "training" with important lore characters such as Tirion or Thassarian would be cool imho.